Over two days of hearings in the U.S. Capitol, CEO and Facebook (FB  ) co-founder Mark Zuckerberg testified before U.S. federal legislators about his company's protection of user data and its business policies. In his two appearances before Congress last week, Zuckerberg attempted to defend Facebook in response to questions regarding his company's data, information, and privacy practices from members of both the House and the Senate.

The news of the Cambridge Analytica data breach, in which the data firm scraped data and accessed information from up to 87 million Facebook users, has damaged Facebook's market value and left many questioning the social media platform's safety and transparency. In the month since the breach was first reported, the social media giant has come under fire for failing to adequately protect its users' information.

In his opening remarks before a joint session of the Senate Judiciary Committee and the Senate Commerce, Science, and Transportation Committee on April 10, Zuckerberg offered an apology for Facebook's failure to keep its tools from being used maliciously. "We didn't take a broad enough view of our responsibility, and that was a big mistake. And it was my mistake. And I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here," said the Facebook head.

© Anthony Quintano / Flickr.com

Despite his apologies, over the course of his congressional hearings, Zuckerberg often dodged lawmakers' toughest questions regarding Facebook's user tracking, its methods of data collection, and its sale of user data to advertisers and data companies. The Facebook CEO also avoided directly answering questions regarding who owns the data and content uploaded to Facebook by users.

In his testimony before the Senate, Zuckerberg acknowledged that his industry may require further regulation, though he emphasized that it ought to be the "right regulation," and agreed to submit proposed measures to Congress.

Zuckerberg was able to avoid the most difficult questions regarding Facebook's user protections and data policies in part because each senator was allotted only five minutes in front of the witness. But when testifying before the House Energy and Commerce Committee on April 11, Zuckerberg faced a tougher environment and harsher lines of interrogation than he had before the Senate. House members, including Representatives Kathy Castor (D-FL) and Ben Ray Luján (D-NM), grilled him repeatedly on Facebook's data collection practices on both users and non-users of the platform. When asked about Facebook's "shadow profiles" of its users, Zuckerberg claimed to be unfamiliar with the term. Since Facebook doesn't use the term "shadow profiles" internally, it's possible that Zuckerberg genuinely was not familiar with the phrase, but it is well established that Facebook maintains such "shadow profiles" on users, which contain information related to the user obtained by Facebook without their permission.

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The Facebook CEO dodged questions about how Facebook treated data from logged-off users, though he acknowledged that the company does track data on users who are not signed in "for security reasons." Zuckerberg further admitted that his company collects data on individuals who have not signed up for Facebook accounts---also "for security purposes."

Zuckerberg's appearance before Congress comes after he refused a similar request to appear before the British Parliament. In the days since the hearings, Facebook has implemented changes to its privacy settings and policies in order to comply with the European Union's newly enacted General Data Protection Regulation (GDPR).