Facebook estimates 126 million people in the US may have seen one of 80,000 Russia-linked posts, while Twitter says Russia-linked automated accounts sent 1.4m tweets about the 2016 election.
Facebook's latest data was included in written testimony provided to US lawmakers, ahead of key hearings with social media and technology companies about Russian meddling in elections on Capitol Hill this week.
Facebook said fake Russian accounts posted 80,000 pieces of content over two years, and the company disabled 5.8 million fake accounts in the United States in October last year.
The company said Russia-linked posts were "simply unacceptable", and it was determined to address the "new threat".
Facebook's general counsel, Colin Stretch, said in the written testimony that the 80,000 posts from Russia's Internet Research Agency were a tiny fraction of content on Facebook, equal to one out of 23,000 posts.
However, the posts violated Facebook's terms of service, and any amount of such activity using fake accounts is too much, Stretch wrote.
"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," he wrote.
The 80,000 posts were published between June 2015 and August 2017. Most of them focused on divisive social and political messages such as race relations, Facebook said.
Meanwhile, Twitter will tell congress it has found 2752 accounts linked to Russian operatives, up from the 201-strong tally it reported in September.
About 1.4 million tweets about the 2016 election were sent by Russia-linked automated accounts - about 0.74 percent of all election tweets - but Twitter said the automated tweets underperformed relative to their volume.
Twitter said it had suspended all 2752 accounts and US congressional investigators had been given the account information.
"State-sanctioned manipulation of elections by sophisticated foreign actors is a new challenge for us - and one that we are determined to meet," Twitter said in written testimony.
Executives from Facebook, Twitter and Google are scheduled to appear before three congressional committees this week on alleged Russian attempts to spread misinformation in the months before and after the 2016 US presidential election.
The Russian government has denied any attempts to sway the election, in which President Donald Trump, a Republican, defeated Democrat Hillary Clinton.