Facebook has informed U.S. investigators that the social network recently discovered it sold advertisements to a Russia-based operation targeting U.S. voters during last year's presidential election.
Facebook revealed the findings in a blog post on September 6 by its chief security officer, Alex Stamos, and said it was cooperating with investigators in Congress and at the Justice Department who are probing reports of Russian attempts to interfere in the U.S. election.
The Washington Post, citing people familiar with the matter, said the social network had sold $100,000 in ads to a Russian "troll farm" that has a history of promoting pro-Kremlin propaganda.
Stamos said Facebook also uncovered $50,000 more in ads clearly of a political nature that might have links to Russia.
Many of the ads placed by the Russian operation promoted 470 "fake accounts" and web pages spreading polarizing views on topics such as immigration, gun rights, race, and gay rights, he said.
Stamos said the ads began in the summer of 2015 and only a small portion of them directly named Republican nominee Donald Trump and Democrat Hillary Clinton. More ads ran in 2015 than during the 2016 election year, he said.
"The vast majority of ads...didn't specifically reference the U.S. presidential election, voting, or a particular candidate," he said. "Rather, the ads...appeared to focus on amplifying divisive social and political messages across the ideological spectrum -- touching on topics from LGBT [lesbian, biisexual, gay, and transgender] matters to race issues to immigration to gun rights."
Facebook has suspended the "inauthentic" accounts that bought the ads, Stamos said, and it has also taken action against "fake accounts" in France, Germany and other countries where elections are being held or were held this year.
"We will no longer allow pages that repeatedly share false news to advertise on Facebook," he said.
Many U.S. political observers said after the November election that they believed ads and fake news stories spread on popular Facebook sites had played a role in swaying voters, prompting Facebook to start a campaign to identify and root out such false stories for the first time in December.
A study by a Washington State University professor found that before the election, among the fake news stories U.S. Facebook users saw were ones saying Pope Francis had endorsed Trump and that a federal agent who had been investigating his opponent Hillary Clinton was found dead.
The university study found that the dead FBI agent story was shared 1,000 times more often than real newspaper stories that were published at the same time.
"To put this in perspective, if you combined the top stories from the Boston Globe, Washington Post, Chicago Tribune, and L.A. Times, they still had only 5 percent of the viewership" of the fake news story on Facebook, Mike Caufield, the professor who authored the study, wrote in a blog post.
With reporting by The Washington Post and Reuters