In an hours-long grilling Tuesday, the questions ranged from the social network's role in the 2016 presidential elections to how the company handles data.
Here are the key moments.
Zuckerberg said Facebook did not notify the Federal Trade Commission about the Cambridge Analytica data leak from 2015 because the company "considered it a closed case."
The profiles of 87 million Facebook users were harvested by a quiz app with the data being sold to political consultancy Cambridge Analytica, which supported Donald Trump in the election. Facebook demanded the maker of the quiz app as well as Cambridge Analytica delete the data. The social network was assured that this was done in 2015, but recent reports suggest that the data still exists. Cambridge Analytica denies this.
Facebook admitted it hadn't checked the data was erased after it made the request to Cambridge Analytica to do so.
"We considered it a closed case. In retrospect that was clearly a mistake. We shouldn't have taken their word for it," Zuckerberg told the senators.
Facebook is "working" with special counsel Robert Mueller's investigation into Russian interference in the 2016 presidential election, according to Zuckerberg. He said he had not been personally interviewed by Mueller's team, but that others in the company had.
"I want to be careful here, because that work is confidential. We are in open session and I don't want to reveal anything that is confidential," Zuckerberg said.
There was a heated exchange between Sen. Lindsey Graham, R-S.C., and Zuckerberg over whether Facebook is a monopoly or not.
Zuckerberg said: "Doesn't feel like that to me."
Confidently. That's the message from Zuckerberg who said he feels the social network is well equipped to handle upcoming votes such as the 2018 U.S. midterms and elections in countries like India and Brazil.
"This is an arms race. They're (potential groups that want to intervene) going to keep on getting better at this. And we need to invest in keeping on getting better at this too," Zuckerberg said.
"I have more confidence that we're going to get this right," he added.
Zuckerberg walked a fine line showing that he was open to regulations, but nothing that would fundamentally blow apart Facebook's business model.
"My position is not that there should be no regulation," Zuckerberg said.
"I think the real question, as the internet becomes more important in people's lives, is what is the right regulation, not whether there should be or not."
Zuckerberg clarified this, saying that "there will always be a version of Facebook that is free." The CEO explained that an ad-supported model is "most aligned with our mission of trying to connect everyone in the world, because we want to offer a free service that everyone can afford."
Facebook has come under fire for not having a clear explanation of how data it collects are being used. Republican Sen. John Kennedy of Louisiana highlighted this point with a frank comment.
"Here's what everyone's been trying to tell you today — and I say it gently — your user agreement sucks," Kennedy said. "The purpose of a user agreement is to cover Facebook's rear end, not inform users of their rights."
Zuckerberg said he imagines that "most people do not read the whole thing," but they have the "opportunity" to.
The debate on whether Facebook is a publisher or merely a technology platform continues and was brought up in the testimony. Sen. Dan Sullivan, R-Alaska, asked Zuckerberg how he classed Facebook.
"When people ask us if we're a media company or a publisher, my understanding is — what the heart of what they're really getting at is — do we feel responsibility for the content on our platform?" Zuckerberg said.
"The answer to that, I think, is clearly yes, but I don't think that's incompatible with fundamentally, at our core, being a technology company where the main thing we do is have engineers and build products."