But he stiffly defended Facebook's use of the data and postings of the 2.2 billion users of its free platform - in order to attract the ad revenue that the $480 billion company depends on. So we need to use that to force Facebook and other social media to take notice of this.
Zuckerberg said he was getting to the bottom of what the UK-based firm did and will tell everyone who may have been affected. In contrast to the information that users (more or less) willingly share, the vast majority of users have no idea that such data even exists.
Facebook is directing people to their settings page where you can update the information you share with apps and websites. Cambridge Analytica then obtained the data and was said to have used it to try to influence elections around the world.
Some analysts said Zuckerberg's appearance suggests a new path forward for social media under closer scrutiny.
His admission that even the company's tech-savvy founder was unable to protect his own information underscored the problem Facebook has in persuading sceptical lawmakers that users can easily safeguard their own information and that further legislation governing Facebook is unnecessary.
Responding to a question, he told lawmakers that he meant to initiate legal action against the firm accused of stealing personal data and using it for political purposes in the 2016 US Presidential elections. As I have said, by the end this year, we are going to have more than 20,000 people working on security and content review across the company. Yet several Republican lawmakers complained to Zuckerberg about what they called a bias on Facebook.
"Every time that someone chooses to share something on Facebook. there is a control".
Congressman Lujan pointed out that if non-users wanted to know what data was collected about them, they get directed to another page and are asked to sign up.
He claimed that "there's a very common misperception. that we sell data to advertisers", adding that "we do not sell data to advertisers". But he said his company "can do a better job of explaining how advertising works".
Wired collated a complete list of all the questions Facebook's CEO said he would have to refer to his team.
"The GDPR requires us to do a few more things and we are going to extend that to the world", he said.
Facebook does limit some content that could be related to "terrorism" but, "We don't think of it as censorship". "We build planes to help connect people, and I don't consider ourselves to be an aerospace company", he said. That goes for fake news, foreign interference in elections and hate speech, as well as developers and data privacy, he said.
After being grilled repeatedly on Day One about his responsibility in enforcing data privacy and review procedures at Facebook, Mark reiterated on Day Two that he was responsible for the actions at Facebook and apologised for his "mistakes", echoing the sentiment from his opening statement on the first day. "I want to make sure in an open session I don't reveal something that's confidential", he said. He had no response when asked how a non-Facebook member could remove information without signing up for the service.