Facebook Inc. on Monday said it estimates 10 million people saw ads it has discovered on its platform paid for by Russian entities, but warned that it may not have uncovered all malicious activity that attempted to interfere in the American political process.

The revelation from Facebook quantifies for the first time the spread of the known Russian activity since the social network said last month it had identified 470 “inauthentic” Russian-backed accounts responsible for $100,000 in advertising spending. Facebook on Monday presented congressional investigators with data on 3,000 ads bought by the Russian actors before and after the U.S. presidential election.

About half of the ads were seen after the election, Facebook said, and one quarter were never shown to anyone. Half of the ads cost less than $3 to run.

“We hope that by cooperating with Congress, the special counsel and our industry partners, we will help keep bad actors off our platform,” Facebook said in a statement Monday.

“What should alarm the American people is the brazen exploitation and distortion of popular opinion by a hostile foreign power, amounting to really an attack on our democracy, to disrupt our election by surreptitiously targeting voters in certain places with certain backgrounds and views,” Sen. Richard Blumenthal (D., Conn.), a member of the Senate Judiciary Committee, said Monday. He added that he viewed some of the ads that were provided to select members of Congress on Monday.

House Intelligence Committee ranking Democrat Adam Schiff (D-CA) Photo: jim bourg/Reuters

The accounts behind the 5,200 Russian-backed ads represented both sides of the political spectrum, and attempted to drive clicks and follows. “Secured Borders,” for example, often railed against illegal immigration, while “Blacktivist” was more aligned with the Black Lives Matters movement. A source with knowledge of the pages confirmed their authenticity.

Facebook’s announcement about the reach of the Russian ads indicates that online information giants may still be in the early stages of uncovering the extent of hidden influence on its platform. Twitter Inc., for example, cross-checked Facebook’s list of 470 Russian-backed accounts to determine if there was Russian-backed activity on the short-messaging platform. That approach drew fire from lawmakers, who said Twitter’s investigation was inadequate. Google, part of Alphabet Inc., is conducting its own extensive internal investigation.

Rep. Adam Schiff, the top Democrat on the House Intelligence Committee said Monday that ads from Facebook and Twitter are likely “the smallest concentric circle of Russian activity and there could very well be a lot more.”

“What Facebook has found thus far have been ads that were funded by money that could be directly tracked back to Russia,” Mr. Schiff (D., Calif.) said. “And I think probably with respect to a certain group operating in Russia. So to the degree Russians funneled money through third countries, that is a whole category that we don’t have an answer to yet.”

Facebook’s admission last month of Russian activity came after it said over the summer it had found no such activity. On Monday, it left the door open to the discovery of more ads. “We’re still looking for abuse and bad actors,” Facebook’s Vice President of Policy and Communications Elliot Schrage said in a statement. “Our internal investigation continues.”

Russia has denied interfering in the election.

Facebook’s estimate of the audience for the Russian ads is lower than some experts anticipated. Joel Yakuel, founder and chief executive of New York-based digital ad firm Agency Within, had said the Russian ads could have reached as many as 20 million or more people on the social network.

Facebook said the ads ran between 2015 and May of this year. One percent of the ads cost $1,000 or more, Facebook said.

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Depending on how they are presented, ads on provocative topics, such as the ones the Russian actors purchased, can have a wide reach at a low cost if the messages go viral or gain traction among their target audience, according to ad buyers. In particular, getting a lot of shares appears to drive down the cost, ad buyers have said. This could explain why the Russian actors paid so little for some of the ads.

Facebook said some of the ads were paid for in Russian currency. Mr. Schrage defended Facebook for not identifying the ads based on currency, “because the overwhelming majority of advertisers who pay in Russian currency, like the overwhelming majority of people who access Facebook from Russia, aren’t doing anything wrong.”

Mr. Blumenthal noted that the ads “don’t advocate for a candidate, and yet they do” because the ads mention hot-topic issues that divide people politically, such as gay marriage or immigration. He called for the ads to be made public.

Write to Georgia Wells at Georgia.Wells@wsj.com and Natalie Andrews at Natalie.Andrews@wsj.com