But why is Zuckerberg in trouble?
Pressure has increased for the company to explain its terms of service and data gathering operations since details about the company's 2016 election cooperation with Cambridge Analytica have become public, and the bulk of Tuesday's session was devoted to questions about privacy.
An audience member also joined in the trolling of Zuckerberg.
Facebook shares soared and ended the day up 4.5 percent as Zuckerberg remained cool, calm and collected during the hearing. His testimony is set to get underway at 2:15 p.m. ET.
Zuckerberg is scheduled to appear before a joint hearing of the U.S. Senate's Commerce and Judiciary committees.
Zuckerberg replied that Facebook was working on it, citing various steps in vogue to tackle hate speech in Myanmar.
The company's delay in identifying Russian efforts to interfere in the 2016 election is "one of [his] biggest regrets in running the company", Zuckerberg said.
Zuckerberg's Tuesday remarks were released by the Senate. It's not enough to just connect people.
His notes, which the AP said were "briefly visible" to one of its photographers, include bulleted lists with headings such as "accountability", "data security", "election integrity (Russia)" and "Defend Facebook".
The social media pioneer admitted "we have made a lot of mistakes" and "I'm sorry", repeatedly making references the company's "origin story" of being set-up in his Harvard dorm room. Tomorrow he will run the gauntlet again before the House committee on energy and commerce.
How has Facebook responded to the Cambridge Analytica scandal?
Facebook had previously accused an academic, Dr Aleksandr Kogan, of violating its terms by passing on data from a personality test app - thisisyourdigitallife - to the UK-based political consultancy firm.
His opening statement did not reveal new information about how data was shared or what Facebook will do. Cambridge Analytica's Facebook account has since been suspended.
Otherwise, Senator Peters used his chance to grill Zuckerberg to voice a few other popular questions users have about how Facebook gathers and stores information, in light of the Cambridge Analytica scandal.