The hearing - before the House Energy and Commerce Committee - has proven just as tense as the Tuesday Senate session. Some lawmakers said that Facebook's recent missteps - on data privacy as well as disinformation, including propaganda from Russia - illustrate that the company and the entire tech industry must be regulated.
Opening the session, the House panel's leader Republican Rep. Greg Walden, Ore., called Facebook an "American success story." But he added: "While Facebook has certainly grown, I worry it has not matured. I think it is time to ask whether Facebook may have moved too fast and broken too many things."
Driving lawmakers' scrutiny is the controversy around Cambridge Analytica, a political consultancy tapped by President Donald Trump's 2016 campaign that improperly accessed the names, "likes" and other personal information of millions of Facebook users. For the first time, Zuckerberg said that his data had been swept up by an app that fed data on 87 million users to Cambridge Analytica.
In the wake of its review of the firm's activities, Facebook also has acknowledged that malicious actors scraped information from the public profiles of practically its entire base, more than 2 billion users. Such scraping heightens the odds that Facebook could be subject to major fines from the Federal Trade Commission, which is investigating the matter.
Zuckerberg started the House hearing by repeating the same apology he gave to the Senate a day earlier. "It was my mistake, and I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here," he told House lawmakers.
But his calm demeanor at times appeared to frustrate lawmakers, who lobbed questions at the 33-year-old billionaire.
In one exchange on Wednesday, Democratic Rep. Anna Eshoo -- a lawmaker who represents a slice of Silicon Valley -- repeatedly needled Zuckerberg for failing to explain its data collection practices to users in "clear and pedestrian language."
Republican Rep. Marsha Blackburn later charged that Facebook looks "a whole lot like the Truman Show," where users' information is "made available to people they don't know, and then that data is crunched and used and they are fully unaware of this."
The Tennessee lawmaker cited laws that govern health data, financial transactions and other industries, before citing her bill that would require tech companies to obtain user permission before they can collect and sell user data. Facebook has long lobbied against the so-called Browser Act.
Lawmakers have said the Facebook leader must provide greater clarity as to exactly how Cambridge Analytica obtained data on 87 million users in the first place. They warned a suit-clad Zuckerberg that tough regulation and scrutiny might follow if Facebook failed once again to improve its business practices.
So far, lawmakers haven't extracted many new concessions from Zuckerberg, even as they warned him that tough regulation -- and even more scrutiny -- might follow if Facebook fails again to improve its business practices.
"If all we do is have a hearing and nothing happens, then that's not accomplishing anything," said New Jersey Rep. Frank Pallone, the top Democrat on the House Energy and Commerce Committee.
A day earlier, Senate lawmakers expressed the same fears.
"Unless there are specific rules and regulations enforced by an outside agency, I have no assurance that these kinds of vague commitments are going to produce action," Democratic Sen. Richard Blumenthal, Conn., said during the Tuesday Senate hearing.
"Mr. Zuckerberg, you've said you're sorry. I appreciate the apologies," added Democratic Sen. Catherine Cortez Masto, Nev., during later questioning. "But please stop apologizing and make the change."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)