Facebook’s Mark Zuckerberg on Friday night posted a message on his Facebook page about so-called fake news and the controversy over his company’s role in carrying it to hundreds of millions of users. “The bottom line is: We take misinformation seriously,” he wrote.

Immediately adjacent to his post when I checked it, at the top of the “Sponsored” column, was a photo of Clint Eastwood and the the headline “Clint Eastwood gone at 84 – ‘In loving memory’ messages pour in from fans around the world,” plus a link to a website with the word “news” in its name. The news was, of course, entirely fake. Clint is fine.

Zuckerberg is in an impossible spot. Since the election he has been under attack based on conjecture that made-up information presented as news, mostly pro-Trump, and circulated on Facebook may have tipped the election’s outcome. This is like blaming AT&T and Verizon for the lies people tell on the phone, but of course it’s also different. Everything on Facebook runs through Facebook servers and can be analyzed by the company, which is critical to its pitch to advertisers that they can target their ads, within limits, based on users’ interests and histories.

So why can’t it stop phony news?

The answer is that the task is impossible. What is news, and what is a news organization, as distinct from what any random human chooses to say on his or her Facebook page? Any of Facebook’s billion-plus users worldwide can set up a page with a plausible-sounding newsy name and start writing any nonsense they like in a journalistic style, and any other user can like it, and then anything can happen. A New York Times case study explains how a completely false report on anti-Trump protests in Austin, Texas, by a man with about 40 Twitter followers was shared more than 350,000 times on Facebook.

No organization could possibly monitor and confirm all such reports, as Zuckerberg realizes. “We do not want to be arbiters of truth ourselves,” he wrote in his post, “but instead rely on our community and trusted third parties.” He shouldn’t have to worry about this at all – getting in the middle of what his users say to each other is arguably the opposite of his proper role – but he’s confronting the reality that he must take some kind of action. He will likely make several changes enabling users to report false information more easily, perhaps inviting third parties to add warnings to stories that appear suspect, blocking ad revenue to repeat offenders, and other actions.

Regardless of what Zuckerberg does, business leaders must prepare for being victimized by fake news. It’s happening already. A headline at a news site called TruthFeed recently quoted PepsiCo CEO Indra Nooyi as telling Trump supporters to “take their business elsewhere.” Trump supporters called for a boycott, the stock fell – but the quote was fabricated. She never said it.

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The words “media” and “news” have taken on strange new meanings, and we all, like Zuckerberg, have to figure out the implications for ourselves.