Time and again, communal hatreds overrun the newsfeed — the primary portal for news and information for many users — unchecked as local media are displaced by Facebook and governments find themselves with little leverage over the company. Some users, energized by hate speech and misinformation, plot real-world attacks.

A reconstruction of Sri Lanka’s descent into violence, based on interviews with officials, victims and ordinary users caught up in online anger, found that Facebook’s newsfeed played a central role in nearly every step from rumor to killing. Facebook officials, they say, ignored repeated warnings of the potential for violence, resisting pressure to hire moderators or establish emergency points of contact.

Facebook declined to respond in detail to questions about its role in Sri Lanka’s violence, but a spokeswoman said in an email that “we remove such content as soon as we’re made aware of it.” She said the company was “building up teams that deal with reported content” and investing in “technology and local language expertise to help us swiftly remove hate content.”

Sri Lankans say they see little evidence of change. And in other countries, as Facebook expands, analysts and activists worry they, too, may see violence.

One Town, Two Versions

Five hours east of Medamahanuwara lies the real Ampara, a small town of concrete buildings surrounded by open green fields. The road there passes over verdant mountains before coasting through tropical flatlands, contested territory during the civil war that ended in 2009, now distinguished mostly by quiet teahouses.

But the imagined Ampara, which exists in rumors and memes on Sinhalese-speaking Facebook, is the shadowy epicenter of a Muslim plot to sterilize and destroy Sri Lanka’s Sinhalese majority.

As Tamil-speaking Muslims, the Atham-Lebbe brothers knew nothing of that version of Ampara when, using money they saved laboring in Saudi Arabia, they opened a one-room restaurant there. They had no way to anticipate that, on a warm evening in late February, the real and imagined Amparas would collide, upending their lives and provoking a brief national breakdown.

It began with a customer yelling in Sinhalese about something he had found in his dinner. Unable to understand Sinhalese, Farsith, the 28-year-old brother running the register, ignored him.

He did not know that the day before, a viral Facebook rumor claimed, falsely, that the police had seized 23,000 sterilization pills from a Muslim pharmacist in Ampara.

The irate customer drew a crowd, which gathered around Farsith, shouting: “You put in sterilization medicine, didn’t you?”mute

He grasped only that they were asking about a lump of flour in the customer’s meal, and worried that saying the wrong thing might turn the crowd violent.

“I don’t know,” Farsith said in broken Sinhalese. “Yes, we put?”

The mob, hearing confirmation, beat him, destroyed the shop and set fire to the local mosque.

In an earlier time, this might have ended in Ampara. But Farsith’s “admission” had been recorded on a cellphone. Within hours, a popular Facebook group, the Buddhist Information Center, pushed out the shaky, 18-second video, presenting it as proof of long-rumored Muslim plots. Then it spread.

Vigilante Justice

As Facebook pushes into developing countries, it tends to be initially received as a force for good.

In Sri Lanka, it keeps families in touch even as many work abroad. It provides for unprecedented open expression and access to information. Government officials say it was essential for the democratic transition that swept them into office in 2015.

But where institutions are weak or undeveloped, Facebook’s newsfeed can inadvertently amplify dangerous tendencies. Designed to maximize user time on site, it promotes whatever wins the most attention. Posts that tap into negative, primal emotions like anger or fear, studies have found,produce the highest engagement, and so proliferate.

In the Western countries for which Facebook was designed, this leads to online arguments, angry identity politics and polarization. But in developing countries, Facebook is often perceived as synonymous with the internet and reputable sources are scarce, allowing emotionally charged rumors to run rampant. Shared among trusted friends and family members, they can become conventional wisdom.

And where people do not feel they can rely on the police or courts to keep them safe, research shows, panic over a perceived threat can lead some to take matters into their own hands — to lynch.

Last year, in rural Indonesia, rumors spread on Facebook and WhatsApp, a Facebook-owned messaging tool, that gangs were kidnapping local children and selling their organs. Some messages included photos of dismembered bodies or fake police fliers. Almost immediately, locals in nine villages lynched outsiders they suspected of coming for their children.

Near-identical social media rumors have also led to attacks in India and Mexico. Lynchings are increasingly filmed and posted back to Facebook, where they go viral as grisly tutorials.

‘You Report to Facebook. They Do Nothing’

In a small office lined with posters in Colombo, Sri Lanka’s capital, members of an advocacy group called the Center for Policy Alternatives watched as hate exploded on Facebook — all inspired by the video from Ampara, which had overtaken Sinhalese social media in just a week.

One post declared, “Kill all Muslims, don’t even save an infant.” A prominent extremist urged his followers to descend on the city of Kandy to “reap without leaving an iota behind.”

Desperate, the researchers flagged the video and subsequent posts using Facebook’s on-site reporting tool.

Though they and government officials had repeatedly asked Facebook to establish direct lines, the company had insisted this tool would be sufficient, they said. But nearly every report got the same response: the content did not violate Facebook’s standards.

“You report to Facebook, they do nothing,” one of the researchers, Amalini De Sayrah, said. “There’s incitements to violence against entire communities and Facebook says it doesn’t violate community standards.”

In government offices across town, officials “felt a sense of helplessness,” Sudarshana Gunawardana, the head of public information, recounted. Before Facebook, he said, officials facing communal violence “could ask media heads to be sensible, they could have their own media strategy.”

But now it was as if his country’s information policies were set at Facebook headquarters in Menlo Park, Calif. The officials rushed out statements debunking the sterilization rumors but could not match Facebook’s influence.

Officials had pleaded with Facebook representatives, in a meeting in October, to better police hate speech and misinformation, which they warned could spiral into violence. They asked the company to establish an emergency point of contact in case it did. In a separate meeting, civic leaders urged Facebook to hire Sinhalese-speaking moderators to staff its reporting tool.

The Facebook employees left offering only vague promises, officials said.

Facebook still appears to employ few Sinhalese moderators. A call to a third-party employment service revealed that around 25 Sinhalese moderator openings, first listed last June, remain unfilled. The jobs are based in India, which has few Sinhalese speakers.

Facebook has no office in Sri Lanka, which officials say makes it difficult to impose regulations.

Mr. Gunawardana, the public information head, said that with Facebook unresponsive, he used the platform’s reporting tool. He, too, found that nothing happened.

“There needs to be some kind of engagement with countries like Sri Lanka by big companies who look at us only as markets,” he said. “We’re a society, we’re not just a market.”

The Thrill of Tribalism

Facebook’s most consequential impact may be in amplifying the universal tendency toward tribalism. Posts dividing the world into “us” and “them” rise naturally, tapping into users’ desire to belong.

Its gamelike interface rewards engagement, delivering a dopamine boostwhen users accrue likes and responses, training users to indulge behaviors that win affirmation.

And because its algorithm unintentionally privileges negativity, the greatest rush comes by attacking outsiders: The other sports team. The other political party. The ethnic minority.

Online outrage mobs will be familiar to any social media user. But in places with histories of vigilantism, they can work themselves up to real-world attacks. Last year in Cancún, Mexico, for instance, Facebook arguments over racist videos escalated to fatal mob violence.

Mass media has long been used to mobilize mass violence. Facebook, by democratizing communication tools, gives anyone with a smartphone the ability to broadcast hate.

Facebook did not create Sri Lanka’s history of ethnic distrust any more than it created anti-Rohingya sentiment in Myanmar.

But the platform, by supercharging content that taps into tribal identity, can upset fragile communal balances. In India, Facebook-based misinformation has been linked repeatedly to religious violence, including riots in 2012 that left several dead, foretelling what has since become a wider trend.

“We don’t completely blame Facebook,” said Harindra Dissanayake, a presidential adviser in Sri Lanka. “The germs are ours, but Facebook is the wind, you know?”

The Boiling Point

As anger over the Ampara video spread online, extremists like Amith Weerasinghe, a Sinhalese nationalist with thousands of followers on Facebook, found opportunity. He posted repeatedly about the beating of the truck driver, M.G. Kumarasinghe, portraying it as further proof of the Muslim threat. Mr. Weerasinghe stoked anger through images overlaid with text.

When Mr. Kumarasinghe died on March 3, online emotions surged into calls for action: attend the funeral to show support. Sinhalese arrived by the busload, fanning out to nearby towns. Online, they migrated from Facebook to private WhatsApp groups, where they could plan in secret.

In a video posted to at least one group, a man dressed as a monk yells, “The sword at home is no longer to cut jackfruit so kindly sharpen that sword and go.”

In another group, a user shared a photo of a dozen makeshift weapons with a list of targets: “Thannekumbura mosque and the mosque in Muruthalawa tonight. Tomorrow supposedly Pilimathalawa and Kandy.”

On Facebook, Mr. Weerasinghe posted a video that showed him walking the shops of a town called Digana, warning that too many were owned by Muslims, and urging Sinhalese to take the town back. The researchers in Colombo reported his video to Facebook, along with his earlier posts, but all remained online.

Over the next three days, mobs descended on several towns, burning mosques, Muslim-owned shops and homes. One of those towns was Digana. And one of those homes, among the storefronts of its winding central street, belonged to the Basith family.

Abdul Basith, a 27-year-old aspiring journalist, was trapped inside.

“They have broken all the doors in our house, large stones are falling inside,” Mr. Basith said in a call to his uncle as the attack began. “The house is burning.”

The next morning, the police found his body.

In response, the government temporarily blocked most social media. Only then did Facebook representatives get in touch with Sri Lankan officials, they say. Mr. Weerasinghe’s page was closed the same day.

One Size Fits All

No organization has ever had to police billions of users in a panoply of languages. Although Facebook prohibits incitement and hate speech, there is no clear line between prudence and censorship.

Despite criticism and concerns from civil society groups, the company has done little to change its strategy of pushing into developing societies with weak institutions and histories of social instability, opening up information spaces where anger and fear often can dominate.

When Facebook entered Myanmar in 2014, Buddhist extremists seized on the platform, spreading misinformation that set off a deadly riot that year. In 2017, hate speech on Facebook contributed to ethnic cleansing against Myanmar’s Rohingya minority.

Amrit Ahuja, a company representative, says Facebook’s approach to hate speech “has evolved” globally. The company plans to hire more moderators and increase coordination with officials and civic groups, she said in an email, to “help keep our community in Sri Lanka safe.”

Adam Mosseri, who runs Facebook’s newsfeed, said on a Slate podcast that he and his team were “losing sleep” over the platform’s role in Myanmar. Tweaks to the algorithm, he said, will privilege people’s long-term interests over their short-term preferences in an effort to address the problem.

Change is not without risk for the company. In January, when Mark Zuckerberg, the Facebook chief executive, announced changes to the newsfeed, the company’s stock tumbled 4.5 percent in a few hours.

And it is not without risks for users. From October to March, Facebook presented users in six countries, including Sri Lanka, with a separate newsfeed prioritizing content from friends and family. Posts by professional media were hidden away on another tab.

“While this experiment lasted, many of us missed out on the bigger picture, on more credible news,” said Nalaka Gunawardene, a Sri Lankan media analyst. “It’s possible that this experiment inadvertently spread hate views in these six countries.”

‘You’re From the Video!’

A week after the violence, Shivnath Thukral, Facebook’s public policy director for South Asia, and two of his colleagues flew to Colombo, for a meeting with a group of government aides.

Mr. Thukral was conciliatory, acknowledging that Facebook had failed to address hate speech and promising better collaboration. In a call with civic leaders, he conceded that Facebook did not have enough Sinhalese moderators, pledging to hire more.

Still, government officials said, they face the same problem as before. Facebook wields enormous influence over their society, but they have little over Facebook.

Even blocking access did not work. One official estimated that nearly three million users in Sri Lanka continued accessing social media via Virtual Private Networks, which connect to the internet from outside the country.

As officials met in Colombo, Atham-Lebbe Farsith, the Muslim restaurant worker, was in hiding. He had shaved his beard. Not to hide his faith, he said, but because even in the Muslim village where he found shelter, he could hardly make it a block without being recognized.

“People would ask me all sorts of questions,” he said. “‘You’re from the video!’”

Facebook had turned him into a national villain. It helped destroy his business, sending his family deeply into debt. And it had nearly gotten him killed.

But he refused to abandon the platform. With long, empty days in hiding, he said, “I have more time and I look at Facebook much more.”

“It’s not that I have more faith that social media is accurate, but you have to spend time and money to go to the market to get a newspaper,” he said. “I can just open my phone and get the news instead.”

“Whether it’s wrong or right, it’s what I read.”