Facebook has found 80,000 posts published by Russia-based operatives that were aimed at swaying U.S. voters, and about 126 million Americans may have viewed those posts over a two-year period.
Facebook's disclosure in written congressional testimony seen by media on October 30 came as Twitter reported in testimony that it also had found far more accounts linked to the same Russian operatives, the Internet Research Agency, known for promoting pro-Moscow messages.
Both companies are due to testify before the Senate Judiciary Committee about their findings on October 31.
Facebook's latest disclosure shows that Russian efforts to influence political opinion in the United States far exceeded the company's previous estimate.
In its written testimony, Facebook said the 80,000 posts it found that were published between June 2015 and August 2017 may have reached half of the U.S. population of voting age.
Facebook said most of the posts, which could have been viewed by voters over Facebook's news feeds or through endorsements and "likes" by other Facebook users, focused on divisive social and political messages such as race relations.
The leading social network said that such "organic" posts that appear in users' news feeds are distinct from more than 3,000 advertisements linked to the Russian agency that Facebook previously disclosed and turned over to congressional committees.
The ads — many of which also focused on divisive social issues such as race and immigration — directed people to click the advertiser's pages, where they could then like or share its material.
"These actions run counter to Facebook's mission of building community and everything we stand for. And we are determined to do everything we can to address this new threat," Facebook General Counsel Colin Stretch said in his written testimony, according to media reports.
Twitter's latest disclosure in written testimony increased by more than 10-fold the number of accounts it found were linked to the same Russia Internet agency, which has been described as a "troll farm."
Twitter said it found 2,752 accounts connected with the agency and it had suspended all of them and given the account names to congressional investigators.
The Russia-linked accounts put out 1.4 million election-related tweets from September 2016 through November 15, 2016, nearly half of them automated, Twitter said.
The company also found nine Russian accounts that bought ads, most of which came from the state-backed news service Russia Today, or RT. Twitter said last week it would no longer accept ads from RT and Sputnik, another Russian state news outlet.
"State-sanctioned manipulation of elections by sophisticated foreign actors is a new challenge for us -- and one that we are determined to meet," Twitter says in the testimony, according to media reports.
Meanwhile, Google announced in a blog post that it found evidence of "limited" misuse of its services by the Russian group, as well as some YouTube channels that were likely backed by Russian agents.
It said that two accounts linked to the Russian group spent $4,700 on ads on its platforms during the 2016 election.
Google said it also found 18 YouTube channels that were likely backed by Russian agents. Those channels hosted 1,108 videos with 43 hours of material, although they racked up just 309,000 views in the United States between June 2015 and November 2016, it said.
"While we have found only limited activity on our services, we will continue to work to prevent all of it, because there is no amount of interference that is acceptable," Google said in the blog post.