Senator Dianne Feinstein (D-California), ranking member of the Judiciary Committee, then made mention of Facebook's lack of transparency after it took more than two years to disclose the "breach of trust" from political research firm CA. Zuckerberg began his testimony by reading his prepared statement, which Congress released on Monday, and said that he takes personal responsibility for Facebook's failure to protect users. Not just from data theft, but also fake news, foreign interference in elections, hate speech and other types of digital abuse.

"We didn't take a broad enough view," Zuckerberg said. "It was my mistake, and I'm sorry. ... I'm responsible for what happens here." He went on to say that while it will take "some time" to fix all of these issues, he's "committed to doing this right."

After Zuckerberg's opening statement, senators asked him a wide range of questions about how Facebook operates, including how it targets users with personalized ads. Senator Bill Nelson (D-Florida) pointed Zuckerberg to what Facebook's chief operating officer, Sheryl Sandberg, said recently about users having to pay if they don't want to see ads on the platform. The CEO said that there will always be a free "version" of Facebook, noting that it's the company's mission to offer a service that's accessible to everyone.

"How do you sustain a business model where users don't pay for your service?" Senator Nelson asked. "Senator, we run ads," Zuckerberg said. He added that if users don't want to see targeted advertisements, there's an option for them to turn that off, noting that even though people generally don't like ads, "people really don't like ads that aren't relevant." Senator Orrin Hatch (R-Utah) said that while there's nothing wrong with offering a free service in exchange for personal data, Facebook needs to be more transparent with users about what type of information it is collecting and what's being done with it.

Naturally, the topic of federal regulation came up, and Zuckerberg pledged to work with the US Senate on proposed regulations. This came up after he said that Facebook had not reported the initial Cambridge Analytica data misuse to the Federal Trade Commission (FTC), which could violate the 2011 settlement agreement between the company and the government agency. At the time, the FTC accused Facebook of deceiving consumers by "telling them they could keep their information private and then repeatedly allowing it to be shared and made public."