After weathering heated questions from two Senate panels, Facebook CEO Mark Zuckerberg returned to Capitol Hill on Wednesday to face more questions from a House committee about the social media platform's transparency and user privacy.
WATCH: Mark Zuckerberg - day 2 of testimony before Congress
Lawmakers want better protections after data breaches that affected tens of millions of users.
WATCH: Facebook hearing
"There was clearly a breach of consumer trust and a likely improper transfer of data," Judiciary Committee Chairman Chuck Grassley, an Iowa Republican, said.
"It was my mistake, and I’m sorry," Zuckerberg said moments later.
Senators demanded action from the Facebook CEO.
"If you and other social media companies do not get your act in order, none of us are going to have any privacy," the top Democrat on the Commerce Committee, Bill Nelson of Florida, said. "If Facebook and other online companies will not or cannot stop the privacy invasions, then we are going to have to — we, the Congress."
Zuckerberg was called to testify after news emerged that the personal data of millions of Facebook users had been harvested without their knowledge by Cambridge Analytica, a British voter profiling company that U.S. President Donald Trump’s campaign hired to target likely supporters in 2016.
Zuckerberg promised to better protect Facebook users. The social media mogul spoke with pride about Facebook’s ability to connect people for the common good but admitted the company has not been proactive in safeguarding its users from misuse of data or those sowing malign messages.
"I started Facebook, I run it. And I’m responsible for what happens here," Zuckerberg said.
Earlier this week, Facebook began notifying 87 million users, most of them in the United States, whose personal data may have been mined by Cambridge Analytica.
Zuckerberg pledged Facebook will scrutinize and, when necessary, block other firms from gaining access to the platform and empower its 2.2 billion users to wall off their apps from third parties.
Senators also sought assurances that Facebook and other social media platforms are blocking fake profiles originating in Russia that spread divisive messages to sow discord during and after the 2016 U.S. election.
"We will be verifying the identity of any advertiser who’s running a political ad," Zuckerberg said. "And we’re also going to do that for [Facebook user] pages … that will make it significantly harder for Russian interference efforts or other inauthentic efforts to spread misinformation through the network."
Vermont Democratic Sen. Patrick Leahy noted that misuse of Facebook extends far beyond the United States, saying that Facebook has been used to spread hate speech against Burma’s Rohingya minority.
"Recently U.N. investigators blamed Facebook for playing a role in inciting possible genocide in Myanmar, and there has been genocide there," Leahy said.
"We’re working on this," Zuckerberg responded. "We’re hiring dozens of more Burmese language content reviewers. Because hate speech is very language-specific, it’s hard to [detect] it without people who speak the local language, and we need to ramp up our effort there dramatically."
Until now, social media companies have been largely self-regulating. Several senators said Congress must consider steps to protect users of the platforms.
"What do we tell our constituents, given what’s happened here, why we should let you self-regulate?" South Carolina Republican Lindsey Graham asked.
"My position is not that there should be no regulation," Zuckerberg said. "I think the real question, as the internet becomes more important in people’s lives, is: What is the right regulation?"
The Facebook CEO promised to submit proposals for regulating social media companies and work with lawmakers to craft legislation.
Facebook faces a backlash from some consumer groups. Members of #DeleteFacebook gathered outside Tuesday’s hearing on Capitol Hill.
"We knew that they had your data, but the extent of what is being breached is a concern for me. What do they know about my children and my grandchildren?" said a woman who identified herself as Alison.
Lawmakers pledged to hold separate hearings focusing on Cambridge Analytica in the near future.