Facebook says a Russian group posted more than 80,000 times on its service during and after the 2016 US election, potentially reaching as many as 126 million users.
The company plans to disclose these numbers to the Senate Judiciary Committee on Tuesday, according to a person familiar with the testimony.
Twitter plans to tell the same committee that it has uncovered and shut down 2,752 accounts linked to the same group, Russia's Internet Research Agency, which is known for promoting pro-Russian government positions.
Facebook, Twitter and Google will testify at three Capitol Hill hearings on Tuesday and Wednesday.
The Russian government has denied any attempts to sway the election, in which President Donald Trump defeated Democrat Hillary Clinton.
Colin Stretch, Facebook's general counsel, plans to tell the judiciary panel that 120 pages set up by the Russian agency posted repeatedly between January 2015 and August 2017. The company estimates that roughly 29 million people were directly "served" posts in their news feeds from the agency over that time.
On Twitter, the Russia-linked accounts put out 1.4 million election-related tweets from September through November 15 last year - nearly half of them automated. The company also found nine Russian accounts that bought ads, most of which came from the state-backed news service Russia Today.
Twitter said last week it would no longer accept ads from RT and Sputnik, another state-sponsored news outlet. It will donate the $US1.9 million ($A2.5 million) it has earned from RT since 2011 to support external research into political uses of Twitter.
Google said two accounts linked to the Russian group spent $US4,700 on ads its platforms during the 2016 election. The company also found 18 YouTube channels 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 US between June 2015 and November 2016.
All three firms are expected to face questions about what evidence of Russian interference they found on their services, as well as why they didn't find it earlier.
