Like North Korea, everybody agrees that fake news is a big problem. But also like the Hermit Kingdom, nobody really knows what to do about it.

Facebook, a site from which a substantial number of people acquire their daily news, has decided that pages that post fake stories will be banned from advertising. That's a perfectly fine decision, but it raises a bigger and more profound question: Who decides which news is fake? Mark Zuckerberg?

The Trouble with Fake News

According to TechCrunch, Facebook collaborates with third-party fact-checkers to verify the veracity of disputed stories. While this is a good first step, it will remain woefully inadequate for three major reasons: (1) Reputable news outlets often publish information that is dubious or flat-out wrong, without ever bothering to correct it; (2) People will never agree on what constitutes fake news; and (3) Activist organizations are, almost by definition, in the business of spreading fake news.

Examples of the first problem could fill several textbooks. The New York Times is perhaps the most notorious offender. America's self-appointed newspaper of record has proven itself to be a veritable sewer pipe of pseudoscience, publishing rants against GMOs and raves for organic food and alternative medicine. The paper gleefully trashes mainstream science in its attempt to cater to its Upper West Side clientele.

Will Facebook punish the New York Times for promoting acupuncture? Or will it dish out justice to MSNBC, after Brian Williams admitted that he believes scaring people is a part of his job? Or what about outlets (like the Daily Mail) that regularly sensationalize the news, such as the ridiculous notion that Nutella causes cancer? It's unlikely.

The second problem is even more difficult: Because of differences in values and priorities, people will never agree on what constitutes "fake news." The TechCrunch article elaborates that phony news isn't "supposed to be stories that are disputed for reasons of opinion or partisanship, but rather outright hoaxes and lies." The problem, though, is that some opinion articles are so lopsided, that they really should be considered lies.

Consider the question, "Is the economy doing well?" That's a deceivingly tricky question to answer. Is economic performance best gauged by the stock market? GDP growth? Unemployment? Inequality? No matter how the question is answered, somebody will think it is wrong. And without a doubt, a journalist or op-ed writer could cherrypick economic statistics to paint whatever picture he wants. In other words, any analysis of the economy could be deemed "fake news."

The third problem may be the worst for Facebook. Many pseudoscientific organizations have pages on the social media site. The Organic Consumers Association promotes both organic food and anti-vaccine myths. The Environmental Working Group publishes false information about pesticides and chemicals. PETA is just plain nuts. These and many other anti-science organizations have large followings (and probably equally large advertising budgets). Will Facebook punish them, too, even if that means turning down money? Highly doubtful.

Quis Custodiet Ipsos Custodes?

It's laudable that Facebook recognizes that it has a moral responsibility to police the content on its website. But the timeless question, "Who will watch the watchmen?" remains frustratingly unanswered. Mr. Zuckerberg, a 33-year-old who believes he can cure all the world's diseases, may not be the best watchman.