“Pope Francis Shocks World, Endorses Donald Trump for President.” “FBI Agent Suspected in Hillary Email Leaks Found Dead of Apparent Murder-Suicide.” “Rush Reveals Michelle's Perverted Past After She Dumps on Trump.” Those headlines didn't come from the New York Times or CNN; they were likely written by teenagers in Macedonia. Those fake news stories were written as clickbait, designed to draw readers to fake-news sites, where the Balkan teens made money by selling ads.

If last fall's election will go down in history as the Election of Unintended Consequences, those fake stories are no exception. They wound up circulating copiously on Twitter and Facebook; on the latter, the top-20 fake stories actually triggered more clicks than the top-20 real ones. Fake news became fodder for ugly partisan warfare online, too. Worst of all, it might have affected the presidential election results. Remember, 44 percent of U.S. adults get their news from Facebook.

You wouldn't think that fake news would be controversial. Surely we all agree that something as important as a presidential election should be based on truth. Can't we just ask Facebook and Twitter to block fake news?

We can, but they can't. The problem isn't technological—it's philosophical. “Identifying the ‘truth’ is complicated,” Facebook CEO Mark Zuckerberg wrote in response to the phenomenon of fake news. “While some hoaxes can be completely debunked, a greater amount of content, including from mainstream sources, often gets the basic idea right but some details wrong or omitted. An even greater volume of stories express opinions that many will disagree with and flag as incorrect even when factual.”

So yes, the headline about the pope was clearly fake. But what about rumor and gossip stories? How can anyone know if they're true? Or what about satire stories from, for example, the Onion and the New Yorker's Andy Borowitz? Neither makes any attempt to deceive, and yet both are often passed around online as fact by people who suffer from, ahem, humor-deficit disorder.

Once fake news became a headline, both Google and Facebook stopped accepting advertising relationships with fake-news sites. There goes the financial incentive of those Macedonian teenagers.

And despite Zuckerberg's initial assertion that it's “extremely unlikely hoaxes changed the outcome of this election,” Facebook is taking more steps to fight the problem—by making it easier to report fake stories, for example, and considering the addition of warning labels to stories that readers have flagged as phony.

But here's the thing. Remember the first time it became possible to assemble customized news pages (like Google News), where you saw news stories pertaining to your interests and nothing more? People worried that we'd never be exposed to stories that we might have stumbled onto when flipping, say, through a newspaper.

Well, the Facebook problem is a thousand times worse. On social media sites, you decide whose posts you want to read. On Facebook, they're your friends; on Twitter, they're people you choose to follow. In both cases, you're following like-minded people, whose opinions you prefer. In other words, you're no longer choosing topics you want to read about; now you're choosing which slant on the news you want to see. You're building your own echo chamber.

All of this helps explain why the “let the community decide” approach to filtering out bogus stories is problematic. For everyone in your echo chamber who flags a story as fake, the parallel universe on the other side of the hyperpartisan divide will mark it as true.

If we ever decide to do this presidential election thing again, the fake news stories will still be around. But three things will be different, all hopeful signs. First, Facebook and Google will have removed the ad-revenue incentive for publishing them. Second, Facebook's planned new policies and algorithms will, at least, screen out some of the deliberately misleading stories.

Above all, we'll be more cynical. Having lived through the first major fake-news election cycle and then spent four years talking about it, maybe we'll be more discerning next time.