In the three months prior to the 2016 presidential election, false news stories favoring Trump were shared 30 million times on Facebook, reports a National Bureau of Economic Research study of how fake news affected the election. Fake news stories favoring Clinton were shared 8 million times.
About 15 percent of adults saw the average fake news headline in the three months before the election, according to a survey fielded by the NBER team. Half of them—or 8 percent—believed the fake story. The NBER researchers calculate that, "for fake news to have changed the outcome of the election, a single fake news story would need to have convinced about 0.7 percent of Clinton voters and non-voters who saw it to shift their votes to Trump, a persuasion equivalent to seeing 36 television campaign ads."
Source: National Bureau of Economic Research, Social Media and Fake News in the 2016 Election, NBER Working Paper #23089 ($5)