Facebook's concession that it sold $100,000 in ads to Russian-linked accounts previous year may be "just the tip of the iceberg" of how social networks were used to interfere in the election, warned the top Democrat on the Senate Intelligence Committee.
In addition to the 470 accounts that appeared to be run from Russia, Mr Stamos said investigators discovered $50,000 in spending on 2,200 ads that "might have originated in Russia", including ads purchased by accounts with IP addresses in the USA but with Russian language settings.
Google said Thursday that it does not have evidence that its advertising platforms were used by a Russian propaganda campaign during the 2016 presidential campaign.
The company deleted accounts and pages involved in buying the ads believed to be tied to Russian Federation because they were fake users in violation of the network's policies.
Senator Mark Warner said Congress may need to update laws in order to make them consistent with rules governing television advertising.
In its January assessment of Russia's election meddling, USA intelligence agencies determined the "likely financier" of the Internet Research Agency's army of "professional trolls located in Saint Petersburg is a close [Russian President Vladimir] Putin ally with ties to Russian intelligence".
Stamos said about one-quarter of the ads were geographically targeted and more ran in 2015 than 2016. There is nothing stopping an ad-buyer from spending thousands of dollars to boost an article from a partisan news outlet that they may in no way be affiliated with.
Facebook was also the target of a 20-minute monologue by the MSNBC host Rachel Maddow on Wednesday night, in which she pointedly noted the company's past denials to media outlets including Time, McClatchy and CNN that it had found any Russian-bought ads.
Facebook has taken a series of new steps since the USA presidential election to crack down on the spread of misinformation on the social network, including launching a third-party fact-checking program and improving artificial intelligence technology that can detect fake accounts.
"This is a very significant set of data points produced by Facebook", Schiff said, adding: "Left unanswered in what we received from Facebook - because it is beyond the scope of what they are able to determine - is whether there was any coordination between these social media trolls and the campaign". A day after the election, he played down the role fake news may have played in the election, calling the theory "pretty insane".
Facebook's Wednesday disclosure was an about-face after months of public denials that Russian money was behind political ads on its platform.
Sen. Mark Warner, the top Democrat on the Senate intelligence committee, told CNN on Wednesday that Facebook's disclosure was just the "tip of the iceberg" when it came to election interference on social media.
But the findings buttress US intelligence agency conclusions that Russian Federation was actively involved in shaping the election.
"I want to see Twitter back in" as well, he said.
In a blog post, Facebook's chief security officer Alex Stamos said that the accounts appeared to be part of an organised campaign to sow disinformation. Facebook said it can't share the ads while the investigation continues. Ex-employees who have gone public with their experiences at the company in Internet postings and in media interviews have said their work entailed creating fake Twitter and Facebook accounts and using them to circulate pro-Kremlin propaganda. The company says its estimates could include non-residents and people who self-report ages that do not match the government's figures.
Foreign nationals or governments are banned under federal law from making contributions or spending money to influence any election in the United States. "And if, in fact, there was foreign money that went into advertising in U.S. elections, that's a Justice Department thing to look at, it's an FEC question, because foreign money can not be used on behalf of elections".