On Friday, August 26, Facebook fired its entire staff of news moderators after accusations of bias against conservative news sources, and decided that its Trending algorithm could handle things fine on its own. On Monday, August 29, the top story on Facebook's Trending news section was a fake story aimed at conservative readers claiming that Fox News host Megyn Kelly had been fired after being outed as a Clinton supporter.
For better or worse, spotting fake stories was the primary reason Facebook had a staff of human news moderators, not (probably*) a nefarious scheme to control the political dialogue. Fake news is endemic on Facebook because the hoaxes rely on gullible people who like to share the stories with their gullible friends. Experienced humans can spot these fakes, but algorithms are really bad at it and only see that a lot of people are sharing it.