Social media sites are where people go to find compatriots and plan attacks like the thwarted attempt last year to kidnap Michigan’s governor. YouTube’s algorithm has radicalized countless youth. And this week’s rioters used right-leaning social media sites like Parler to pass around directions to the Capitol that would help them avoid police detection.

So we have to ask Facebook and Twitter: Is this who you really want to be? A safe space for riot-inciting, lying public officials, who nonetheless bring in lots of online engagement?

Mr. Zuckerberg said on Wednesday in an internal note that the company would increase its moderation of the president’s account because of the “emergency” of the day’s mob violence. That’s a telling admission that Facebook hadn’t already been devoting enough resources to moderating Mr. Trump’s dangerous account.

Jan. 6, 2021, ought to be social media’s day of reckoning.