Facebook has said as many as 126 million American users may have seen content uploaded by Russia-based operatives over the past two years.
The social networking site said about 80,000 posts were produced before and after the 2016 presidential election.
Most of the posts focused on divisive social and political messages.
Facebook released the figures ahead of a Senate hearing where it, together with Twitter and Google. will detail Russia’s impact on the popular sites.
Russia has repeatedly denied allegations that it attempted to influence the last US presidential election, in which Donald Trump beat Hillary Clinton.
In a separate development on Monday, an investigation by independent counsel Robert Mueller into possible collusion between the Trump campaign and Russia produced its first charges against two former aides.
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It also emerged that a third former aide had pleaded guilty in early October to lying to the FBI.
President Trump has dismissed allegations of collusion with Moscow, and has repeatedly called on Mrs Clinton to be investigated.
What is Facebook saying?
Facebook says some 80,000 posts were published between June 2015 and August 2017 and were seen by about 29 million Americans directly, according to a draft of prepared remarks seen by US media ahead of Tuesday’s Senate judiciary committee hearing.
These posts, which Facebook says were created by a Kremlin-linked company, were amplified through likes, shares and comments and spread to tens of millions more people.
Facebook also said it had deleted 170 Instagram accounts, which posted about 120,000 pieces of content.
“These actions run counter to Facebook’s mission of building community and everything we stand for,” wrote Facebook’s general counsel Colin Stretch.
“And we are determined to do everything we can to address this new threat.”
In a blog post from earlier this month, Facebook’s Elliot Schrage said that many of the posts did not violate the company’s content policies.
They were removed, he said, because they were inauthentic – the Russians behind the posts did not identify themselves as such.
What defines reach?
A post counts as reaching someone when it’s shown in Facebook News Feeds, so this figure takes no account of the number of people who may or may not have actually read it.
We do not know therefore how many of those 126 million people actually consumed the 80,000 posts and consequently how many of these were in influential in terms of affecting the mindsets of voters.
Clearly, a mass targeting of posts can have a subliminal impact on people but it is hard to evaluate with any certainty based on the data we have seen so far.
What about other social networking sites?
Google also revealed on Monday that Russian trolls had uploaded more than 1,000 political videos on YouTube on 18 different channels.
The company said they had very low view counts and there was no evidence they had been targeting American viewers.
Meanwhile, Twitter found and suspended all 2,752 accounts that it had tracked to the Russia-based Internet Research Agency, Reuters quotes a source as saying.
These accounts, which have now been suspended, posted about 131,000 tweets between September 2016 and November 2016.
All three companies are also due to testify on Wednesday at the Senate and House intelligence committees.
