“I think it’s pretty much impossible, I believe, to start a company in your dorm room and then grow it to be at the scale that we’re at now without making some mistakes,” Mr. Zuckerberg said.
Senator Richard Durbin, Democrat of Illinois, zeroed in on the central issue of the hearing, asking Mr. Zuckerberg whether he would be comfortable sharing aloud the name of the hotel where he stayed on Monday night, or whether he would be comfortable sharing the names of the people he has messaged this week.
“No. I would probably not choose to do that publicly here,” Mr. Zuckerberg said.
“I think that may be what this is all about,” Mr. Durbin said. “Your right to privacy. The limits of your right to privacy. And how much you give away in modern America in the name of, quote, connecting people around the world.”
Mr. Zuckerberg was the only technology chief in the room, but he was often treated as a stand-in for the whole industry. Facebook has come under intense criticism for the Cambridge Analytica leak and for its initial response, which set off a #DeleteFacebook campaign online and sent the stock plunging more than 15 percent.
But the hearing was about more than Facebook; it exposed a critical turning point as the power, sophistication and potential exploitation of technology outpaces what users, regulators or even its creators expected or seem prepared to handle.
The moment is creating a showdown between two national power centers — Washington and Silicon Valley — as they jockey in a technology-centric world. Although Washington has long served as a check on the power of Wall Street and other profitable industries, lawmakers have tended to act as cheerleaders for technology companies rather than watchdogs. Light regulation enabled a culture of freewheeling innovation, and the beloved products that Silicon Valley companies created made them politically convenient allies.
Today, five of the eight largest companies in the world are West Coast technology companies. Only a single East Coast institution, JPMorgan Chase, cracks the top 10. And lawmakers on both sides of the aisle suggested that Facebook and other companies may not be able to police themselves.
“Have you gotten too big?” Senator Dan Sullivan, Republican of Alaska, asked Mr. Zuckerberg, before suggesting that Facebook might need to be reined in to protect it from itself.
Senator Lindsey Graham, Republican of South Carolina, pressed Mr. Zuckerberg on whether Facebook had become a monopoly, asking why Congress “should let you self-regulate?”
Mr. Zuckerberg said he welcomed some form of regulation, as long as it was the “right regulation.” He also expressed support for the Honest Ads Act, a bill in Congress that would require more disclosures from online political advertisers.
His answers did not mollify lawmakers, including Mr. Graham, who said in a statement after the hearing that “continued self-regulation is not the right answer when it comes to dealing with the abuses we have seen on Facebook.”
Senator Richard Blumenthal, Democrat of Connecticut, said in an interview that he was “unsatisfied” and that it was clear Facebook could not and would not fully regulate itself and that Congress needed to provide a solution.
“The old saying: There ought to be a law,” he said. “There has to be a law. Unless there’s a law, their business model is going to continue to maximize profit over privacy.”
Mr. Zuckerberg’s appearance before more than 40 senators came after weeks of preparation, and it appeared to pay off, as he seemed calm, deferential and prepared. Mr. Zuckerberg offered humor about the company’s onetime mantra: “move fast and break things.” (It has since been edited to “move fast with stable infrastructure.”) He insisted on continuing questions when offered a break, eliciting smiles and laughter from staff sitting behind him. However, when Mr. Zuckerberg did take a break, he left behind his notes, which were quickly photographed and contained talking points for various topics including “Defend Facebook,” “Disturbing Content” and “Election integrity (Russia).”
His performance won accolades on Wall Street. “This is a different Mark Zuckerberg than the Street was fearing,” said Daniel Ives, chief strategy officer and head of technology research for GBH Insights in New York. “It’s a defining 48 hours that will determine the future of Facebook, and so far he has passed with flying colors, and the Street is relieved.”
But Mr. Zuckerberg acknowledged that his idealistic view of humanity, and those of his fellow executives, had exposed Facebook’s roughly 2.2 billion users to danger. Among them: Facebook failed to detect and stop Russian interference in the 2016 presidential election, an oversight that Mr. Zuckerberg called “one of my greatest regrets.”
Senator John Thune, Republican of South Dakota and chairman of the Senate Commerce Committee, called Facebook and its role in society “extraordinary” and began the hearing by explaining that Facebook and Mr. Zuckerberg were being singled out because of the company’s power.
Mr. Thune said the Cambridge Analytica situation underscored how Facebook could be used for nefarious reasons, saying it appeared “to be the result of people exploiting the tools you created to manipulate users’ information.”
In an indication that he may support legislation for internet companies, Mr. Thune said, “In the past, many of my colleagues on both sides of the aisle have been willing to defer to tech companies’ efforts to regulate themselves. But this may be changing.”
Mr. Zuckerberg was careful when pressed on how he defined the company, wary of opening it up to new legal liability. He said that Facebook was responsible for the content on the social network, but he argued that it was a technology company, rather than a publisher.
“I agree that we’re responsible for the content,” Mr. Zuckerberg said. “But we don’t produce the content.”
The primary thing that Facebook does, he added, is employ engineers and build products.
Some of the hardest questions came from Senator Kamala Harris, Democrat of California, whose name is being floated for a presidential bid. Ms. Harris pressed Mr. Zuckerberg on whether Facebook executives made a decision not to inform users about the Cambridge Analytica data leak when they learned in 2015 that data was sold by a researcher to the political consulting firm. The question was crucial to the Federal Trade Commission’s investigation of Facebook’s violation of a 2011 agreement to protect users’ privacy. If the company withheld information, which violates its agreement, the company could face record fines.
Mr. Zuckerberg did not admit that the company explicitly decided to withhold that information from consumers, but he said the company had made a mistake in not informing users.
There were glimmers of a partisan divide during the hearing. Senator Ted Cruz, Republican of Texas, asked about Facebook’s handling of conservative media, including content related to Glenn Beck and a Fox News personality; Democrats questioned Mr. Zuckerberg on how quickly Facebook responded to Russian meddling in the election.
But the dominant theme of the day was uncertainty about how to deal with Facebook, a complex, multifaceted giant that even technologists have struggled to define. Lawmakers — many of whom grew up in an era without social media — labored at times to understand the fine-grain nuances of Facebook’s business model, such as the difference between selling user data to advertisers and allowing advertisers to target ads to an aggregated slice of Facebook users. At one point, Mr. Zuckerberg was forced to shoot down a conspiracy theory floated by Gary Peters, Democrat of Michigan, that Facebook listens to users through their microphones in order to serve them ads.
Mr. Zuckerberg, who spent weeks being coached for the hearing by a team of outside experts, remained cool and collected while fielding question after question.
But the lawmakers’ technical sophistication was mostly irrelevant — by the end of the session, the warning had been sent: In order to avoid a much harsher regulatory glare that could include major punitive measures, Facebook needs to stop apologizing, get its act together, and show that it is capable of changing.
“This episode has clearly hurt us,” Mr. Zuckerberg said. “We have to do a lot of work about building trust back.”
An earlier version of this article misstated the message written on cardboard cutouts of Mark Zuckerberg, the chief executive of Facebook, outside the Capital Building on Tuesday. The message was “Fix Fakebook,” not “Fix Facebook.”