Mr. Stamos wrote that while some of the ads specifically mentioned the two candidates, most focused instead on issues that were polarizing the electorate: “divisive social and political messages across the ideological spectrum — touching on topics from LGBT matters to race issues to immigration to gun rights.”
Facebook did not make public any of the ads, nor did it say how many people saw them. But Mr. Trump regularly offered outspoken comments on those issues during the campaign, denouncing “political correctness” and rallying his supporters on the right.
In its review of election-related advertising, Facebook said it had also found an additional 2,200 ads, costing $50,000, that had less certain indications of a Russian connection. Some of those ads, for instance, were purchased by Facebook accounts with internet protocol addresses that appeared to be in the United States but with the language set to Russian.
In a January report, the Federal Bureau of Investigation, Central Intelligence Agency and National Security Agency concluded that the Russian government, on direct orders from President Vladimir V. Putin, was responsible for hacking Democratic targets and leaking thousands of emails and other documents in an attempt to hurt Mrs. Clinton’s campaign and mar her reputation.
The report also found that hundreds of Russian “trolls,” or paid social media users, had posted anti-Clinton messages. But it did not name Facebook or address the question of advertising.
The January intelligence report said the “likely financier” of the Internet Research Agency was “a close Putin ally with ties to Russian intelligence.” The company, profiled by The New York Times Magazine in 2015, is in St. Petersburg and uses its small army of trolls to put out messages supportive of Russian government policy.
The revelations can only add to the political skirmishing in Washington over Russia’s role in the election. Mr. Trump has often dismissed the Russian hacking story as “fake news” and bristled at any implication that Mr. Putin had helped him win. To date, while news reports have uncovered many meetings and contacts between Trump associates and Russians, there has been no evidence proving collusion in the hacking or other Russian activities.
Representative Adam B. Schiff of California, the ranking Democrat on the House Intelligence Committee, said in a telephone interview that the Facebook disclosure “certainly quantifies the Russian use of at least one social media platform with a level of granularity that we did not have before.” He said the committee has been in touch with Facebook for some time, adding, “I don’t think this is the last word on the matter by Facebook or in terms of our investigation on the social media issue.”
Mr. Schiff said he has more questions for Facebook, including when the company first become aware of the problem, what warning signs it found, how sophisticated the Russian operation was and what steps Facebook was taking to guard against such activity in the future.
“Clearly Facebook doesn’t want to become the arbiter of what’s true and what’s not true,” Mr. Schiff said. “But they do have a civil responsibility to do the best they can to inform their users of when they’re being manipulated by a foreign actor.”
The suspicion that Russia had a hand in placing Facebook ads was first mentioned in a Time magazine article in May, but Wednesday’s announcement was the company’s first acknowledgment of the problem.
Facebook, which offers a sophisticated level of targeting to advertisers, has been in the center of a storm over the role that it played in propagating false news reports and other misleading information during the campaign. The company acknowledged in April that fake accounts were a problem and said it accepted the intelligence agencies’ findings on the matter, but it avoided naming Russia.
Mr. Stamos’s post on Wednesday was more forthright, saying that the fake Facebook accounts connected to the ads “likely operated out of Russia.”
After initially denying that fake news on the service had any influence on the election, Facebook’s chief executive, Mark Zuckerberg, has gradually come around to the notion that the company must do more. Facebook has implemented a series of steps to combat fake content, including recruiting outside reviewers to check out and flag dubious articles.
But the new measures do not directly affect Facebook ads. Advertisers pay to have particular Facebook posts displayed high in the news feeds of whatever group of people is targeted.
The audience for an ad can be chosen using broad factors, such as middle-aged American men, or very specific ones, such as mothers who live in Minneapolis and like churches and the Minnesota Twins.
That ability to target is valuable to political campaigns, and the company actively reaches out to candidates around the world to teach them how to use Facebook to get their messages out, including through paid advertising.
One question underlying the investigation of possible collusion between the Trump campaign and Russia is whether Russia-sponsored operators would have needed any guidance from American political experts. Facebook said that some of the ads linked to Russian accounts had targeted particular geographic areas, which may raise questions about whether anyone had helped direct such targeting.
Under federal law, foreign governments, companies and citizens are prohibited from spending money to influence American elections. Facebook’s disclosure could add an additional element to the possible crimes under investigation by Mr. Mueller.