Facebook's concession that it sold $100,000 in ads to Russian-linked accounts last 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.

Virginia Sen. Mark Warner, who is leading the Senate's investigation into Russia's election attack, said Thursday he has long believed that Moscow used overt social media sites like Facebook and Twitter to intervene in the 2016 election, as well as other covert tools such as cyberattacks.

"And you know, the first reaction from Facebook, of course, was, "Well, you're crazy, nothing's going on,'" Warner said at a national security conference in Washington, D.C.

"Well, we find yesterday there actually was something going on. And I think all we saw yesterday in terms of their brief was the tip of the iceberg."

Facebook acknowledged in a blog post on Wednesday that 470 accounts "affiliated with one another and likely operated out of Russia" purchased around 3,000 ads between June 2015 and May 2017.

The majority of the ads did not reference the presidential campaign or a particular candidate but instead focused "on amplifying divisive social and political messages across the ideological spectrum," the social network said.

The topics targeted in the ads included LGBT issues, race matters, immigration and gun rights. Influence-watchers say one technique that foreign powers use to try to shape the American information environment is to amplify divisions that already exist.

And Facebook plays an outsize role in delivering news to Americans — 67 percent of American adults use the site and 44 percent of U.S. adults get news on it, according to a Pew Research Center survey from 2016.

At the same time, $100,000 worth of ads is a drop in the bucket compared with the billions of dollars in business that Facebook does every year. It isn't clear whether the strategy represented an experiment by the Russian-linked buyers that they abandoned or how much more of the "iceberg" might be revealed in records of Facebook or other such platforms.

In its blog post, Facebook said about 25 percent of the suspect ads were "geographically targeted," and most of those ran in 2015.

Warner said he wants the committee to meet with representatives of Facebook, Twitter, Google and other online giants to see what can be done to prevent social media sites from being used as tools by foreign adversaries to meddle in future U.S. elections.

"I think Americans, when it particularly comes to elections, ought to be able to know if there is a foreign-sponsored content coming into their electoral process," he said.

What investigators in Washington also want to know is whether any Americans, especially those within the Trump orbit, might have played any role in advising Russians on where to target their influence operations within the U.S.

One hypothesis is that Trump's digital operation might have used its own polling or other information to single out areas where it could most benefit from help.

Members of Congress and others are also looking down the road: Warner's counterpart on the House Intelligence Committee, California Democratic Rep. Adam Schiff, also addressed the question of how to combat possible Russian interference on Thursday.

"The best protection that we can have is somehow forging the consensus we didn't have last year: that no matter who it may help or who it may hurt, if any foreign power intervenes in our affairs, let alone our elections, they will be repudiated," Schiff said. "And anyone who tries to take advantage of it will be repudiated. More than anything else, I think that's what we need to defend ourselves."

Schiff appeared to be alluding to the willingness of some in the Trump campaign, including Donald Trump Jr., to meet with a Russian delegation that had offered damaging information on Hillary Clinton.

President Trump has brushed off criticism of his son for taking the meeting in June of 2016. But Republicans and Democrats alike have said it was highly inappropriate to meet with the Russians under such circumstances.

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