Their story keeps changing.
First, Facebook founder Mark Zuckerberg scoffed at the notion that disinformation spread on his platform swayed the 2016 presidential election. He called that a "crazy idea."
Later, faced with some uncomfortable revelations, Zuckerberg changed his tune and apologized.
More recently, Facebook fessed up to taking $100,000 for ads from murky but probably Russian-linked sources apparently trying to influence the election. That is, of course, a tiny amount of money, but a very dangerous precedent, especially because of the way those ads reportedly were targeted to undecided voters in swing states.
Now, the social-media giant is scheduled to tell Congress that at least 126 million Americans were exposed to free content from Russian-linked sites during the campaign.
Much of that content was expressly designed to widen the cultural divides in the United States, to drive wedges among its citizens — and in so doing, to help elect the Russian government's preferred candidate: Donald Trump.
And while those more recent numbers are astonishing, the reality is probably far worse.
Research from Columbia University's Tow Center suggests that Russian-linked information — or disinformation — was shared hundreds of millions of times on Facebook.
The numbers boggle the mind.
We need to admit the obvious: If there had been no Facebook spreading Russian propaganda, there might well be no President Trump. And there's little reason to think this kind of influence will wane, that this kind of result won't keep happening.
"The informational underpinnings of democracy have eroded," was how Alexis Madrigal put it in a sweeping Atlantic magazine piece recently.
And Peter Eckersley of the civil liberties group the Electronic Frontier Foundation told The Washington Post on Monday: "We're going to have to figure out what it means for this private social network to be democratically accountable."
But what does that suggest?
Government oversight of social media sounds like a terrible idea, given free-speech considerations. Regulation could easily veer into politically driven censorship.
And Facebook itself, though it claims to be working hard on the problem, hasn't earned anyone's trust. Its ever-shifting acceptance of what happened — coupled with its notorious opaqueness — doesn't inspire confidence.
So what is to be done? Some meaningful steps are necessary — but each solution that is contemplated brings its own set of dangers. And some very tough questions.
Where does sensible regulation turn into censorship?
Where do free-speech and privacy concerns — often at odds with each other — find a reasonable meeting place?
Do we really want the federal government dictating what social-media companies can and can't do?
Tim Wu, author of "The Attention Merchants," offered a radical solution: Facebook should become a nonprofit organization — a public benefit company — thus separating its ruling motivations (profit-making through advertising) from its outsize influence on our society.
It's hard to know just how that would come to pass — and it's hard to imagine it happening in this lifetime — though it seems worth considering.
In the meantime, the government may need to act, at least by exerting oversight.
That's the reluctant conclusion reached by Lincoln Caplan, an author and scholar at Yale Law School, who wrote in Wired magazine: "As it stands, the country's libertarian conception of free speech is allowing, and even ferociously feeding, an erosion of the democracy it is supposed to be essential in making work — and some government regulation of speech on social media may be required to save it."
But we seem to lack even the language or basic facts to discuss this rationally. And meanwhile, Facebook — whose active users already number more than 2 billion worldwide — only grows.
"The way we think and talk about different types of messages has been well and truly broken," Emily Bell, who directs the Tow Center, wrote this week.
The world-changing results of the 2016 election may seem like a one-off — a completely bizarre turn of events that could never be repeated. But the social-media influences that played such a huge role are still functioning today and will be in the near future.
I have little confidence that Facebook will fix itself. And I have even less confidence in the federal government to do the job.
There must be another solution out there. And finding it needs to be an urgent mission.
For more by Margaret Sullivan visit wapo.st/sullivan