Facebook and Twitter have revealed the full reach of Russia-linked political posts in the lead-up to the US presidential election.
Facebook said that as many as 126 million Americans may have seen around 80,000 posts that were published by operatives based in Russia.
The posts were produced before and after the 2016 US presidential election - many with divisive political messages.
Facebook's lawyer Colin Stretch said the posts were a tiny fraction of the social media website's content - around one in every 23,000 posts.
Mr Stretch says the material was posted by Russia's Internet Research Agency between June 2015 and August 2017.
He said: "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."
The details were released as part of written testimony given to US lawmakers and seen by Reuters ahead of Facebook's appearance - along with Google and Twitter - before three congressional committees this week.
The committees are investigating alleged Russian attempts to spread misinformation in the months before the election – claims the Russian government have denied.
Meanwhile, Twitter has told Congress it found 2,752 accounts linked to the same Russian agency, many more than the 201 it initially reported in September.
The new estimate comes after Democratic senator Mark Warner criticised the company's first investigation into the matter as insufficient.
Twitter has suspended the accounts and passed on their details to congressional investigators.
There were also 1.4 million automated tweets about the election - around 0.74% of all election tweets.
In its written testimony, Twitter reportedly said: "State-sanctioned manipulation of elections by sophisticated foreign actors is a new challenge for us - and one that we are determined to meet."