âIâm disappointed that youâre here, and not your C.E.O.s,â said Senator Angus King, independent of Maine.
Lawmakers also complained that the companies had taken months to acknowledge Russiaâs interference on their sites.
âI have more than a little bit of frustration that many of us on this committee have been raising this issue since the beginning of this year, and our claims were, frankly, blown off by the leadership of your companies,â said Senator Mark Warner of Virginia, the top Democrat on the Senate Intelligence Committee.
Facebookâs chief executive, Mark Zuckerberg, and chief operating officer, Sheryl Sandberg, spent Wednesday at the companyâs headquarters in Menlo Park, Calif., talking to investors and analysts as they reported blockbuster quarterly earnings. The stocks of both Google and Facebook, which faced the most criticism in the hearings, are at record highs.
During the earnings call, Mr. Zuckerberg was unequivocal in his stance on the issue of Russian meddling in the election.
âIâve expressed how upset I am that the Russians used our tools to sow mistrust,â Mr. Zuckerberg said, noting that Facebookâs profits will probably be affected by the amount of money the company will spend fighting abuse of its platform. Facebook said it plans to double the number of content reviewers it employs, to 20,000, and will try to add a greater degree of transparency into its advertising system.
âWhat they did is wrong, and weâre not going to stand for it,â Mr. Zuckerberg said.
The tech companies also provided new numbers on the reach of Russiaâs influence campaign. Facebook said an estimated 150 million users of its main site and its subsidiary, Instagram, were exposed to the posts, a larger figure than it provided even as recently as Monday.
During the last of the three hearings, members of the House Intelligence Committee spoke in front of posters displaying the content, complaining that it was divisive.
Representative AndrÃ© Carson, Democrat of Indiana, said an account called Being Patriotic, which amassed 200,000 followers, pushed out content that âcynically exploits grieving officers and their loved ones in order to pit Americans concerned about our law enforcement personnel against Americans concerned about African-American lives lost during police encounters.â
The account was created by the Kremlin-backed Internet Research Agency.
âMy concern is that a dictator like Vladimir Putin abused flaws in our social media platforms to inject the worst kind of identity politics into the voting decisions of at least 100 million Americans,â Mr. Carson said, referring to the Russian president.
Facebook has found a particularly vocal set of critics on the Congressional Black Caucus, of which Mr. Carson is a member along with Representative Terri A. Sewell, Democrat of Alabama. Together, they pressed Facebook to grapple with its role in promoting racial animus.
Ms. Sewell cited figures showing few blacks in Facebookâs work force and its leadership â a lack of diversity that she said made it hard to believe that those reviewing socially divisive ads could spot problematic posts.
The hearings exposed a growing rift between Silicon Valley and Washington, where sentiment toward big tech companies has drastically shifted.
While the lawyers showed humility and promised to beef up security and improve technology to prevent foreign interference in elections, they admitted they could not guarantee they would prevent future intrusions. Googleâs general counsel, Kent Walker, said the company would work on creating new technologies to detect foreign actors and misinformation on its site. All three said they would build artificial intelligence tools to combat fake and problematic content.
Some lawmakers used the hearings to stake a position on the influence of the Kremlinâs social media use in the election. The conclusions, particularly among senators, split along political lines. Republicans offered an implicit defense of the legitimacy of President Trumpâs victory and dismissed the effect of Russian meddling.
âA lot of folks, including many in the media, have tried to reduce this entire conversation down to one premise: Foreign actors conducted a surgically executed covert operation to help elect a United States president,â said Senator Richard M. Burr, Republican of North Carolina and the chairman of the Intelligence Committee. âIâm here to tell you this story does not simplify that easily.â
Senator Marco Rubio, Republican of Florida, emphasized that the real intent of Russian propaganda was to broadly spread misinformation and create chaos.
âThese operations â while weâre talking about the 2016 presidential race â theyâre not limited to 2016, and they were not limited to the presidential race, and they continue to this day,â he said. âThey are much more widespread than one election.â
Such comments offered a rare view into the Senate committeeâs investigation, which has largely played out over the past nine months in secured briefing rooms. Publicly and in private, Mr. Burr and Mr. Warner have taken pains to preserve bipartisan comity, and their success has set the committee apart from the other panels investigating Russiaâs efforts.
But the difference in their emphasis on Wednesday also underscored the political realities buffeting their work. In advancing an investigation tied to Mr. Trump, Mr. Burr has been careful to make clear that the committeeâs work is larger than an individual candidate, and he has repeatedly tried to tamp down expectations about what it might find.
Democrats did not have such reticence.
âWhether the Russians and the campaign coordinated these efforts, we do not yet know, but it is true that the Russians mounted what could be described as an independent expenditure campaign on Mr. Trumpâs behalf,â said Representative Adam B. Schiff of California, the top Democrat on the House Intelligence Committee.
Senator Martin Heinrich, Democrat of New Mexico, also took aim at Mr. Trumpâs dismissal of the role of Russia-linked social media in his win. Mr. Heinrich challenged Colin Stretch, Facebookâs general counsel, to acknowledge such content and the role that fake accounts linked to Russia and other misinformation had in the election.
âLast month, President Trump called Russian-purchased Facebook ads a âhoax,ââ Mr. Heinrich said. âIâve looked at those Russian-sponsored Facebook ads. I certainly hope youâve had a chance to review them. Are they, in fact, a hoax?â
Mr. Stretch said no. âThe existence of those ads were on Facebook,â he said, âand it was not a hoax.â