Facebook told congressional investigators Wednesday that it sold about $100,000 in political advertising -- roughly 3,000 ads -- to Russian troll farms from June 2015 to May 2017.
But Warner, the top Democrat on the Senate intelligence committee, said that he heard a different story from the social media giant during the 2016 election.
"It appeared to me that the very social media sites that we rely on for virtually everything -- our Facebooks, Googles and Twitters -- it was my belief the Russians were using those sites to intervene in our elections," Warner said Thursday, speaking at the Intelligence & National Security Summit in Washington. "And the first reaction from Facebook was: 'Well you're crazy, there's nothing going on' -- well, we find yesterday there actually was something going on."
Foreign influence on social media is just one area that Warner and the Senate intelligence committee are probing as part of their investigation into Russian election interference. Warner said that he wants the committee to hear more from Facebook, Twitter and other social media companies about Russian efforts.
In an interview Wednesday with CNN's Erin Burnett on "OutFront," Warner noted the difference between Facebook's response to the US election in 2016 and the French election in 2017, when Facebook took down 50,000 accounts.
"I think Facebook is learning along the way," the Virginia Democrat said. "I think it really raises a series of questions about a number of social media firms -- and we've got to talk to Twitter as well -- about making clearer about public disclosure. I think the public needs to know what kind of misinformation and disinformation might be appearing on their Facebook news feed or their Twitter news feed."
On Thursday, Warner suggested that Congress could play a role by passing legislation to put disclosure requirements on social media advertising similar to those for television commercials.
"An American can still figure out what content is being used on TV advertising. ... But in social media there's no such requirement," Warner said. "There may be a reform process here. I actually think the social media companies would not oppose, because I think Americans, particularly when it comes to elections, ought to be able to know if there is foreign-sponsored content coming into their electoral process."
Both Warner and California Rep. Adam Schiff, the top Democrat on the House intelligence committee, said at Thursday's conference they have concerns Russia will try yet again to hack US elections.
"There is no software patch for what happened last year, there is no cyberdefense capable enough," Schiff said. "If Russians want to get into the Democratic National Committee in 2020, they'll get in. If they want to get into the Republican National Committee, they will get in."
The two Democrats expressed concern in particular about Russian efforts to hack state election systems. Federal officials have confirmed 21 states were potentially targeted but have not provided details.
"It drives me crazy that we've had 21 states that were broken into and we've still had almost a Kafka-esque response that we can't share with the top (state) election officials because they don't have appropriate clearances," Warner said. "We've got to be better prepared."