“It’s disappointing and disheartening to see how an outside actor can pinpoint areas of division in America and then exploit them,” said Brent Leatherwood, the former executive director of the Tennessee Republican Party. “You think we’re getting to a better place and then something like this throws a Molotov cocktail into it.”

Perhaps no form of communication has ever established itself so quickly and so thoroughly as social media. Hundreds of millions of people around the world have grown to rely on it for news and information. Now Twitter and Facebook are facing a moment of reckoning. They, as well as Google, are being called to account for their role in the deception and chicanery that has surrounded the 2016 campaign, especially from accounts linked to Russia.

How much damage did those accounts do in the months leading up to the presidential election? No one knows, not even the companies themselves, which are slowly and grudgingly releasing data about what happened. Next week, they will send executives to testify at congressional hearings, the beginning of an attempt to calculate an answer.

Google and Facebook are powerful and wealthy companies that are skilled enough to ride out this controversy. But for Twitter — influential, yet smaller and far less financially successful — the situation is more vexing. Its devotion to open discourse is drawing an abundance of troublemakers who threaten to drive out the well-intentioned.

Many of Twitter’s users have long been frustrated with the site’s scattershot efforts at preventing abuse and harassment. Twitter’s critics have counted at least a half-dozen times over the last few years when the company has said it was taking an issue seriously but little seemed to change.