From the Columbia Journalism Review:

Digital-journalism veteran David Cohn has argued that the network’s main purpose is not information so much as it is identity, and the construction by users of a public identity that matches the group they wish to belong to. This is why fake news is so powerful.

“The headline isn’t meant to inform somebody about the world,” wrote Cohn, a senior director at Advance Publications, which owns Condé Nast and Reddit. “The headline is a tool to be used by a person to inform others about who they are. ‘This is me,’ they say when they share that headline. ‘This is what I believe. This shows what tribe I belong to.’ It is virtue signaling.”

Twitter suffers from a similar problem, in the sense that many users seem to see their posts as a way of displaying (or arguing for) their beliefs rather than a way of exchanging verifiable news. But Facebook’s role in the spread of misinformation is orders of magnitude larger than Twitter’s: 2 billion monthly users versus 330 million.