After four years in which social media has been viewed as an antisocial force, the crisis is revealing something surprising, and a bit retro: Twitter, Facebook, YouTube and others can actually deliver on their old promise to democratize information and organize communities, and on their newer promise to drain the toxic information swamp.

The question, which I put to Mark Zuckerberg, chief executive of Facebook, in an interview Thursday, is why it took a global health crisis for them to do so.

Mr. Zuckerberg said the difference between good and bad information is clearer in a medical crisis than in the world of, say, politics.

“When you’re dealing with a pandemic, a lot of the stuff we’re seeing just crossed the threshold,” Mr. Zuckerberg said. “So it’s easier to set policies that are a little more black and white and take a much harder line.”