It has become right-wing conventional wisdom that the January 6 insurrection was not the work of Trump supporters but of antifa and BLM agitators. The New York Times reports that this all started with a tweet from a right-wing radio host named Michael Brown:
Only 13,000 people follow Mr. Brown on Twitter, but his tweet caught the attention of another conservative pundit: Todd Herman, who was guest-hosting Rush Limbaugh’s national radio program. Minutes later, he repeated Mr. Brown’s baseless claim to Mr. Limbaugh’s throngs of listeners: “It’s probably not Trump supporters who would do that. Antifa, BLM, that’s what they do. Right?”
....By day’s end, Laura Ingraham and Sarah Palin had shared it with millions of Fox News viewers, and Representative Matt Gaetz of Florida had stood on the ransacked House floor and claimed that many rioters “were members of the violent terrorist group antifa.”
This is a good example of how social media works. Its direct reach in this case was tiny: 13,000 followers is nothing, and they probably shared the tweet mostly with other true believers who are already so far down the rabbit hole that they hardly matter.
But social media also acts as a kind of laboratory for more conventional media. Most of the really outrageous stuff stays buried in the nether regions of Facebook and Reddit, but occasionally one of the big guns decides to amplify a likely looking conspiracy theory. This time it was Rush Limbaugh's program, followed quickly by Fox News. From there it took on a life of its own.
That's how this stuff works. There are exceptions here and there, but for the most part there's surprisingly little evidence that social media has very much real-world impact on political views. It becomes important only when something that's swirling around the fever swamps gets picked up by media figures with truly vast audiences. This usually means Fox News, where it reaches millions of people and acquires the patina of reliability. After all, a news program wouldn't lie about something like this, would it?

By itself, social media isn't generally all that harmful. Its conspiracy theories mostly get shared within a bubble of other true believers and never make it to the outside world. It's only when something gets picked up by Fox News that it takes on a life of its own. This is why we should worry less about Facebook and more about the real threat to democracy. That's Fox News, and it always has been.