The Congressional hearings come almost a month after news broke that Cambridge Analytica, a data firm with ties to President Donald Trump's campaign, accessed information from as many as 87 million Facebook users without their knowledge.
Congresswoman Anna Eshoo, reading questions from her constituents at a hearing of the House Energy & Commerce Committee, asked Mr Zuckerberg whether his data was "included in the data sold to the malicious third parties". It seems clear Facebook is going to have to unlock that data, because as Lujan stated "We've got to fix that".
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.
"In general we collect data on people who are not signed up for Facebook for security purposes", Zuckerberg said. One of them, Representative Ben Lujan, came down hard on Zuckerberg.
In his testimony, Zuckerberg also stated that Facebook does not see the messages of acquired company WhatsApp, India's most popular messaging platform.
"One of my greatest regrets in running the company is that we were slow in identifying the Russian information operations in 2016", he said. When we first contacted Cambridge Analytica, they told us that they had deleted the data.
He parried questions of how much control people have over their data on the world's largest social media network without a major gaffe, while avoiding being cornered into supporting new government regulation. We're guessing he's distracted by the billions of dollars Facebook earns trading in information it gathers from us, information we might naively have thought was private.
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.More news: Trump says all is 'very calm' at White House
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As many people know, Facebook tracks everything that you do inside the Facebook app and on a Facebook page when you are logged into the social media.
Although much of the current global discussion about internet privacy is focused on Facebook's recently publicized policies and failures, the Gallup poll also found a greater number of Google users are concerned about their protecting their privacy when using the platform-35 percent are "very concerned", a 10-point jump from 2011. "I want to make sure in an open session I don't reveal something that's confidential", he said.
Mark has previously mentioned that he believes that the social media industry needs to be regulated, and he reiterated that view on being questioned by Rep. Fred Upton. Others asked the CEO about Facebook's restriction of a page belonging to pro-Trump vloggers "Diamond and Silk".
"I think we may be updating it a little bit. We build planes to help connect people, and I don't consider ourselves to be an aerospace company", he said.
"We struggle with continuously and the question of what is hate speech versus what is legitimate political speech", Zuckerberg said in response. And it's now a big enough thing that people now have to take this seriously.
"I think he's sorry he has to testify in front of Congress", Kelly said.