Facebook founder and CEO Mark Zuckerberg faced off against senators Tuesday, acknowledging growing pains as his company evolved from a dorm-room project to internet colossus that now faces a crisis of confidence after mishandling users’ data and seeing its platform abused by Russian operatives who meddled in the 2016 U.S. presidential election.
Mr. Zuckerberg was contrite, offering repeated apologies and assuring lawmakers he wants his company to live up to the socially conscious mission he laid out when he founded Facebook as a way to connect college friends.
But lawmakers told him the days of a self-regulating internet dominated by large companies with information monopolies may be coming to an end, with the public demanding a bargain among the government, social media giants and their users.
“I don’t want to have to vote to regulate Facebook, but by God, I will. That depends on you,” said Sen. John Kennedy, Louisiana Republican, challenging Mr. Zuckerberg to right things on his own.
Mr. Kennedy suggested a first step would be to toss the company’s attorneys out of the room and go to work rewriting Facebook’s user agreement — a lump of legalese that Mr. Zuckerberg confirmed he doubted most users bothered to read.
“Your user agreement sucks,” Mr. Kennedy concluded.
The hearing, before the joint Senate Judiciary and Commerce committees, brought some revelations.
Mr. Zuckerberg confirmed that Facebook is working with special counsel Robert Mueller on his investigation into Trump campaign ties to Russian operatives during the presidential race. He even at first said Facebook had received a subpoena, but then recanted, saying he wasn’t sure about that but still confirming that the company is sharing information.
He declined to give more details.
With roughly 2 billion users worldwide, the firm played a central role in the election meddling saga, and Mr. Zuckerberg acknowledged hundreds of Russian-backed accounts were used to try to influence voters. But he insisted the company is already better at weeding out those accounts and is in much better shape heading into this year’s midterm elections.
Mr. Zuckerberg’s appearance — to be followed by testimony to a House panel on Wednesday — marked his first major defense since he acknowledged last month that millions of users had their data mined by a Facebook-approved developer. British university professor Aleksadr Kogan then sold the data to Cambridge Analytica, a political consulting firm with ties to the Trump campaign, which used the data to reach voters based on their profiles.
Initially Facebook has said about 50 million users were affected. That estimate has now risen to 87 million.
Mr. Zuckerberg, while accepting the public thrashing delivered by senators, said his company didn’t break its rules by letting Mr. Kogan mine the data. He said the breach came when Mr. Kogan sold the data to Cambridge Analytica, and then when both Mr. Kogan and the company kept the data even after assuring Facebook in 2015 that it had been deleted.
The youthful Facebook CEO struggled, though, to explain why they didn’t alert the 87 million users at the time that their data had been sold in violation of the company’s rules.
“We considered it a closed case,” he said. “In retrospect, that was clearly a mistake.”
Mr. Zuckerberg sat for about five hours of questions, perched atop a cushion to give him a better position at the desk.
The 33-year-old Mr. Zuckerberg, the youngest man at the hearing, was the sole witness, and respecting the gravity of the day, he had traded his usual gray T-shirt and hoodie for a navy suit and blue tie. He is now worth about $64 billion because of his company.
He was composed and at times convivial with senators, and his performance appeared to steady his company, whose stock price dropped after last month’s revelations. Facebook ended the day up 4.5 percent in trading on the Nasdaq stock exchange.
The most striking moment of Tuesday’s hearing was when Sen. Richard J. Durbin, Illinois Democrat, kicked off his questions by asking if Mr. Zuckerberg would be “comfortable sharing with us the name of the hotel you stayed in last night.”
Mr. Zuckerberg paused, then muttered, “Um, uh, no.”
Mr. Durbin continued: “If you messaged anybody this week, would you share with us the names of the people you’ve messaged?”
“Senator, no, I would probably not choose to do that publicly here,” Mr. Zuckerberg replied.
“I think that might 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.’”
Some senators said they were reluctant to pursue legislation to force better practices on social media.
Sen. Orrin G. Hatch, Utah Republican and the chamber’s senior lawmaker, defended Facebook’s business model of selling access to advertisers, saying users are getting a service out of the platform.
“Did any of these individuals ever stop to ask themselves why Facebook and Google don’t charge for access? Nothing in life is free. Everything involves trade-offs,” he said. “These great websites that don’t charge for access — they extract value in some other way. And there’s nothing wrong with that, as long as they’re upfront about what they’re doing.”
But he was in the minority. Far more senators — and nearly half the Senate was present for the hearing — bounced ideas off Mr. Zuckerberg about what a law would look like.
“If Facebook and other online companies will not or cannot fix these privacy invasions, then we will,” said Sen. Bill Nelson, Florida Democrat.
Mr. Zuckerberg found himself dispelling a number of misperceptions lawmakers had about Facebook’s operations.
He said contrary to belief, Facebook doesn’t sell anyone’s data. Advertisers pay to reach certain audiences, which Facebook identifies and then targets with the ads, but the data behind the targeting is not shared, he said.
The company also will delete all data of a user who closes out an account, and users are able to see exactly what Facebook has stored.
Mr. Zuckerberg also shot down reports that the company listened to calls or recorded from phone microphones. He also said reports that it collected data from other applications outside of Facebook is misleading.
But he did confirm that users of Android smartphones can sync their phone text messaging system with Facebook’s message system, which makes all of the texts accessible to Facebook.
He said that is done with the clear consent of users, who have to opt in to the sharing — something he said is true every step of the way, for most of the privacy issues being raised.
“We do require permission to use the system, and to put information in there, and for all the uses of it,” he said. “I want to be clear. We don’t sell information. So regardless of whether we could get permission to do that, that’s just not a thing that we’re going to go do.”
He repeatedly pushed back on suggestions from senators that his company, by allowing developers to mine data, had violated a settlement it reached in 2011 with the Federal Trade Commission to better protect users’ privacy.
He said the problem was the way developers — in this case Mr. Kogan — breached the terms of the deal they had with him.
“The system basically worked as it was designed. The issue is that we designed the system in a way that wasn’t good,” he said.
Mr. Zuckerberg also admitted that he worries Facebook could become too slanted to the left, and said he is aware Silicon Valley, where his company has headquarters, is an “extremely left-leaning place.”
He said he is committed to keeping Facebook a neutral platform for all ideas.
Sen. Ted Cruz, Texas Republican, ticked off a number of complaints from conservatives who have had pages or posts blocked, or who said stories about conservatives were given lower play. He asked Mr. Zuckerberg if he could think of any page that had been blocked for Planned Parenthood or Moveon.org, two prominent leftist groups, or for any Democratic candidates.
“I’m not sure,” Mr. Zuckerberg said.
“A great many Americans are deeply concerned Facebook and other tech companies are engaged in a pervasive pattern of bias and political censorship,” Mr. Cruz said.