CHAPEL HILL, N.C. — Donald J. Trump’s supporters were probably heartened in September, when, according to an article shared nearly a million times on Facebook, the candidate received an endorsement from Pope Francis. Their opinions on Hillary Clinton may have soured even further after reading a Denver Guardian article that also spread widely on Facebook, which reported days before the election that an F.B.I. agent suspected of involvement in leaking Mrs. Clinton’s emails was found dead in an apparent murder-suicide.

There is just one problem with these articles: They were completely fake.

The pope, a vociferous advocate for refugees, never endorsed anyone. The Denver Guardian doesn’t exist. Yet thanks to Facebook, both of these articles were seen by potentially millions of people. Although corrections also circulated on the social network, they barely registered compared with the reach of the original fabrications.

This is not an anomaly: I encountered thousands of such fake stories last year on social media — and so did American voters, 44 percent of whom use Facebook to get news.

Mark Zuckerberg, Facebook’s chief, believes that it is “a pretty crazy idea” that “fake news on Facebook, which is a very small amount of content, influenced the election in any way.” In holding fast to the claim that his company has little effect on how people make up their minds, Mr. Zuckerberg is doing real damage to American democracy — and to the world.