But a reconstruction of how Facebook-based misinformation and hate speech contributed to anti-Muslim riots in Sri Lanka last month, along with research on how people use social media, suggests that those who set out to be provocateurs are not the only danger — or even the biggest one.

Everyday users might not intend to participate in online outrage, much less lead it. But the incentive structures and social cues of algorithm-driven social media sites like Facebook can train them over time — perhaps without their awareness — to pump up the anger and fear. Eventually, feeding into one another, users arrive at hate speech on their own. Extremism, in other words, can emerge organically.

We saw this firsthand in the small town of Digana, Sri Lanka, a week after anti-Muslim mobs had torn through.

One of the posts seen as inspiring the attacks was a Facebook video posted by Amith Weerasinghe, who has gained a large online following. Just before the mob arrived, he’d filmed himself walking Digana’s shops, warning that too many were owned by Muslims and calling on Sinhalese to take the town back.

But Mr. Weerasinghe’s online stardom was newfound and, according to neighbors, a mere persona.

“He’s from the area, he went to school here,” Jainulabdeen Riyaz, a member of the local Muslim community, said, laughing at the absurdity of this standoffish local boy posing as a crusading outsider. “His father is a carpenter. He’s a normal person.”