The embattled billionaire CEO – accompanied by his wife, pediatrician Priscilla Chan – paid courtesy calls to the two chairmen and two top Democrats of the Senate Commerce and Judiciary Committees, both set to hold a rare joint hearing.

As Zuckerberg, clad in a suit and dark blue tie instead of his usual tee-shirt, made the rounds Monday for more than five hours – each meeting lasting well over an hour and one lasting nearly two – the House Energy and Commerce Committee released his opening statement far ahead of his Wednesday appearance there. A Facebook spokesperson said Zuckerberg’s Senate testimony Tuesday will mirror his Wednesday House appearance.

Lawmakers want Zuckerberg to explain reports that the data firm Cambridge Analytica allegedly misused Facebook data obtained from user profiles.

“It’s clear now that we didn’t do enough to prevent these tools from being used for harm as well,” Zuckerberg plans to tell them, according to his prepared remarks, expressing contrition yet again for allowing third parties to harvest the data without the consent of Facebook’s users. “That goes for fake news, foreign interference in elections, and hate speech, as well as developers and data privacy. 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. I started Facebook, I run it, and I’m responsible for what happens here.”

Sen. Bill Nelson, D-Fla., said Zuckerberg told him during their one-on-one meeting that the company had been “lied to” by Cambridge Analytica when officials there told Facebook that they had gotten rid of all the user data they had collected through a research app, after the company banned those data collection methods.

“When he said to me very forthrightly, ‘We were lied to and we should have caught that,’ I believe that, but I think in today's world that's naïve,” Nelson said.

Cambridge Analytica did work for the Trump campaign during the 2016 presidential election, but a spokesperson for the Trump campaign told ABC News in a statement that the campaign used the RNC for its voter data and not Cambridge Analytica.

Cambridge Analytica did not respond to ABC News’ attempts to get comment on Nelson’s remarks Monday. Nor did it respond to request for comment last week in response to Zuckerberg agreeing to appear before the committees.

Cambridge Analytica has said previously it was unaware the data was improperly obtained by a third party and that it was destroyed as soon as it was made aware. The company also said the data was never used as part of the firm’s work with the Trump campaign.

Nelson told reporters that Zuckerberg walked him through the steps that Facebook has said it will take to combat nefarious actors on the site in the future, including using a third party to verify if users are fake or legitimate.

“If we don’t rein in the misuse of social media, none of us are going to have any privacy anymore,” Nelson warned.

Ahead of the two hearings, Facebook – reeling from negative news stories about alleged misuse of its platform – has unveiled a flurry of fixes and has even intimated that it could live with some form of regulation, something the company has fought against for years.

Starting Monday, Facebook users were supposed to get alerts that indicate if they are one of up to 87 million people whose private information ended up in the hands of a voter data targeting company, Cambridge Analytica, the same company hired by the Trump campaign and other Republicans.

Political and issue ads will now be clearly identified on screen, and users will also have access to information regarding the funding behind those ads – all ahead of the 2018 midterm elections. Advertisers will first have to be approved by the social media company, although that vetting process remains unclear.

Critics say the apology and the actions by the company are just not enough.

“It’s past time for Facebook to come clean about how it is handling user data. We don’t want more apologies, we want information and accountability — and not just in connection with Cambridge Analytica,” Electronic Frontier Foundation’s Legal Director Corynne McSherry told ABC News. “Congress should be asking Facebook for a detailed account of what data it has shared with third parties, what it has done to protect misuse of that data, what it told users about how it would handle their information, and what steps it will take in the future to respect users' privacy rights.”

For the 33-year old chief executive, Tuesday’s testimony before Congress will be a first, and he has been preparing for weeks with a team of hired experts, according to The New York Times. The cascade of policy changes at the company – coming out ahead of hearings – gives Zuckerberg something concrete to tout, as he seeks to show lawmakers that he is the man who can right the ship.

Experts have coached Zuckerberg on how to respond to members, setting up mock hearings with staff and outside advisers who role-play members of Congress, according to The New York Times.

Zuckerberg recently told reporters that he and his staff are in “a never-ending battle,” though critics have pointed to the company’s business model as the root problem – a model that relies on user data to target advertising – to say the current problems are likely to last much longer.

But Zuckerberg, according to the released testimony, plans to stand firm on that.

“My top priority has always been our social mission of connecting people, building community and bringing the world closer together,” Zuckerberg’s prepared remarks say. “Advertisers and developers will never take priority over that as long as I’m running Facebook,” he said in his prepared remarks.

Sen. John Thune, the Republican chairman of the Senate Commerce Committee said, "I think more than anything else right now most of our members want to express their frustration on behalf of their constituents about what’s happened to date and want to hear very seriously about how he intends to fix it."