Facebook believes about 10 million Americans saw targeted ads purchased by a group affiliated with the Russian government designed to sow political divisions ahead of the 2016 US presidential election.
The company disclosed the new information after turning over the roughly 3000 ads over to congressional committees investigating Russia's interference with the election.
The ads appear to have come from a Russian entity known as the Internet Research Agency.
Facebook didn't release the ads publicly, but has said previously the ads weren't tied to individual candidates. The company described most of the ads as focusing on "divisive social and political messages across the ideological spectrum".
The subjects of the ads spanned issues including LGBT, race, immigration and gun rights and many of them encouraged people to follow Facebook Pages on these issues.
Adam Schiff, the ranking member of the House Intelligence Committee, said he hopes to release "a representative sampling" of the ads when tech firms testify at a public hearing before Congress. He also said he was "committed to making all of these ads public as soon as possible".
On Monday, the world's biggest social platform announced plans to hire an additional 1000 employees to review ad submissions, along with other new policies and procedures designed to improve the transparency of Facebook's advertising.
Facebook's role in the controversy over Russian meddling in the American election has put the Silicon Valley giant and its business practices in a harsh spotlight -- and elicited an attack from President Donald Trump, who tweeted that "Facebook has always been anti-Trump".
Facebook chief Mark Zuckerberg responded with a defence of the company's platform as a place for political discourse.