The testimony, heard by members of the Senate Judiciary and Commerce Committees, was prompted by the scandal over Cambridge Analytica's harvesting of Facebook data to influence the 2016 presidential campaign.
"I believe it's important to tell people how data is used, that's why every time a photo is shared there's a control about who you're going to be sharing it with," Zuckerberg told Senator Chuck Grassley, before explaining that Facebook doesn't outline how data is used in the terms of service because "if you make it long and spell out all the detail, then you're probably going to reduce the number of people who will read it."
The CEO informed the politicians that the company is going through a "broader philosophical shift of how we view our responsibility."
"For the first 10 or 12 years of the company I viewed our responsibility as primarily building tools," Zuckerberg told Senator John Thune. "What I think we've learned now across a number of issues… is that we need to take a more proactive role and broader view of our responsibility."
The former Harvard student explained that Facebook hadn't informed the Federal Trade Commission (FTC) about the Cambridge Analytica slipup because it believed that "it was a closed case" once the firm was told to delete the harvested information. Zuckerberg told Senator Bill Nelson that "in retrospect we should've done a full audit."
The hearing later turned toward Russia's alleged interference of the 2016 presidential election. When it comes to foreign actors dipping their toes into US elections, Zuckerberg stated that his team is working to improve artificial intelligence tools to better identify "fake accounts that are spreading false information."
"One of my greatest regrets is that we were slow in identifying the Russian information operations in 2016. We expected them to do more traditional cyber attacks, which we did identify, but we were slow to identifying types of new operations," Zuckerberg stated. "Nature of these attacks though is that there are people in Russia whose job it is to try and exploit our systems and other systems."
"This is an arms race; they're going to keep getting better at this and we need to invest in keeping better at this, too," he added, noting that Facebook would be employing 20,000 people to flag content.
Admitting to knowing about Cambridge Analytica since 2015, Zuckerberg told Senator Dianne Feinstein that Facebook failed to act sooner because the firm wasn't using the social media engine's services.
"We didn't have anything to ban," he told the senator.
The New York native stated that no one within Facebook had been fired because of the Cambridge Analytica scandal. He added that he does not see the firm's harvesting as a violation of Facebook's agreement with the FTC.
Regarding whether or not the perfect regulation exists to prevent a Cambridge Analytica repeat, Zuckerberg urged that he would consult with his team to brainstorm some suggestions for the committees.
Though the first two hours the testimony focused on explaining how Facebook collects and shares data, Zuckerberg between questions stated that Facebook is working with the office of special counsel Robert Mueller. However, he indicated that he was unsure whether or not the company had received any subpoenas.
It was also noted that there is a slight concern over political bias at Facebook, however, there is an active push to "root it out."
Zuckerberg told politicians that Facebook would not "proactively" work with the Trump administration to conduct extreme vetting of immigrants, and said Facebook would only cooperate only if threat of harm was imminent or if it became legally obligated to do so.
An hour before his testimony, Zuckerberg took to the social media site to state that he was going to testify "about how Facebook needs to take a broader view of our responsibility — not just to build tools, but to make sure those tools are used for good."
"I will do everything I can to make Facebook a place where everyone can stay closer with the people they care about, and to make sure it's a positive force in the world," he added.
Once on the scene, the tech mogul was met with hundreds of spectators, many calling for the end of his cash cow.
Zuckerberg's Tuesday showdown is just one of two congressional hearings. The 33-year-old will speak to officials on Wednesday before the House Committee on Energy and Commerce.