Mark Zuckerberg will appear before US lawmakers on Tuesday and Wednesday after placing the blame for privacy and security lapses at Facebook on himself.
The social network's chief executive said he was too idealistic and failed to grasp how the platform — used by two billion people — could be abused and manipulated, according to remarks released by a congressional panel.
He will testify before senators on Capitol Hill amid a firestorm over the hijacking of data on millions of Facebook users by the British firm Cambridge Analytica, which worked with Donald Trump's campaign.
On Monday, Mr Zuckerberg — who is usually dressed in a casual T-shirt — was in a suit and tie as he made the rounds with his assistant Andrea Besmehn for private meetings with lawmakers ahead of the hearings — a key test for the Facebook founder.
"We didn't take a broad enough view of our responsibility, and that was a big mistake. It was my mistake, and I'm sorry," Mr Zuckerberg said in his written testimony released by the House commerce committee.
"I started Facebook, I run it, and I'm responsible for what happens here."
In his written remarks, Mr Zuckerberg called Facebook "an idealistic and optimistic company", adding: "We focused on all the good that connecting people can bring."
But he acknowledged that "it's clear now that we didn't do enough to prevent these tools from being used for harm as well. That goes for fake news, foreign interference in elections, and hate speech, as well as developers and data privacy."
Mr Zuckerberg said he has called for more investments in security that will "significantly impact our profitability going forward", adding: "I want to be clear about what our priority is: protecting our community is more important than maximising our profit."
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Mr Zuckerberg recounted a list of steps announced by Facebook aimed at averting a repeat of the improper use of data by third parties like Cambridge Analytica, and noted that other applications were also being investigated to determine if they did anything wrong.
"We're in the process of investigating every app that had access to a large amount of information before we locked down our platform in 2014," said MR Zuckerberg.
"If we detect suspicious activity, we'll do a full forensic audit. And if we find that someone is improperly using data, we'll ban them and tell everyone affected."
After meeting with Mr Zuckerberg on Monday, Senator Bill Nelson told reporters that Mr Zuckerberg appears to be taking the matter seriously.
"I believe he understands that regulation could be right around the corner," Mr Nelson said.
Mr Nelson said lawmakers would be looking at other social media sites in determining any new regulations.
"It's not just Facebook," the Florida senator said.
Facebook "happens to be the point of the spear, but all these other app sites that get your personal data, that's another way of us losing our privacy," Mr Nelson said.
Facebook has taken a series of proactive steps to make up for massive lapses in protecting user data, as lawmakers signalled they intend to get tough on privacy.
Last week, the company announced new privacy tools to be in place in user news feeds on Monday, and said it would notify the 87 million users affected by the data hijacking scandal, amid probes underway on both sides of the Atlantic.
Over the weekend, it said it had suspended another data analysis firm, US-based Cubeyou, after reports that it had used private data harvested from psychological testing apps for commercial purposes. It also suspended the Canadian firm AggregateIQ over apparent collaboration with Cambridge Analytica.
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On Friday, Facebook sought to quell some concerns over political manipulation of its platform by announcing support for the "Honest Ads Act" that requires election ad buyers to be identified, and to go further with verification of sponsors of ads on key public policy issues.
Mr Zuckerberg said the change will mean "we will hire thousands of more people" to get the new system in place ahead of US midterm elections in November.
"We're starting this in the US and expanding to the rest of the world in the coming months," Mr Zuckerberg said.
On Monday, Facebook agreed to supply proprietary data for a study on its role in elections and democracy.
"The focus will be entirely forward looking. And our goals are to understand Facebook's impact on upcoming elections — like Brazil, India, Mexico and the US midterms — and to inform our future product and policy decisions," Facebook said in a statement.
Facebook has said it has seen little impact on its business from the privacy scandal despite a #deleteFacebook movement and concerns from advertisers.