The near five-hour testimony was far tougher than Tuesday's proceedings with lawmakers of the House Energy and Commerce Committee questioning Facebook's fundamental business model, the way it uses data and what kind of regulation may be appropriate.
Here are the key moments.
Democrat congresswoman Anna Eshoo asked Zuckerberg whether his own data was "included in the data sold to malicious third parties," to which the Facebook CEO responded, "yes".
A quiz app created in 2014 harvested the Facebook profiles of users and their friends before sending the data over to political consultancy Cambridge Analytica. A full timeline of the story is here.
The short answer is no. Zuckerberg seemed startled when lawmakers asked whether his business model would change as a result of the recent scrutiny. Congresswoman Eshoo asked Zuckerberg if he is willing to change Facebook's business model "in the interest of protecting individual privacy."
Zuckerberg said that he is continuing to make changes but was cut off by the congresswoman who reiterated her question.
"Congresswoman, I'm not sure what that means," Zuckerberg responded, in one of the most heated moments of the hearing.
Rep. Frank Pallone, D-N.J., then asked Zuckerberg if he could commit to change all user default settings to minimize "to the greatest extent possible" the collection and use of user data.
"Congressman, this is a complex issue that I think is — deserves more than a one-word answer," Zuckerberg said.
"Well, again, that's disappointing to me, because I think you should make that commitment," Pallone replied.
"I think that it is inevitable that there will need to be some regulation. So my position is not that there should be no regulation. But I also think that you have to be careful about what regulation you put in place," Zuckerberg said.
His comments mirror what he said at Tuesday's hearing.
Rep. Raul Ruiz, D-Calif., asked whether there should be an entity "tasked with overseeing how consumer data is being collected, shared and used" and which could offer guidelines for internet firms to make sure their business practices don't violate the law.
Zuckerberg said that "it's an idea that deserves a lot of consideration."
Lawmakers took Zuckerberg to task on the fact he appeared to deflect many questions, often saying that his team would follow up. Rep. Debbie Dingell, D-Mich., said
"As CEO, you didn't know some key facts. You didn't know about major court cases regarding your privacy policies against your company. You didn't know that the FTC doesn't have fining authority and that Facebook could not have received fines for the 2011 consent order. You didn't know what a shadow profile was. You didn't know how many apps you need to audit," the congresswoman said.
"You did not know how many other firms have been sold data by Dr. Kogan other than Cambridge Analytica and Eunoia Technologies, even though you were asked that question yesterday. And yes, we were all paying attention yesterday. You don't even know all the kinds of information Facebook is collecting from its own users."
Dingell then asked how many "like" buttons there are on non-Facebook webpages.
Zuckerberg said he didn't know the answer but said "we'll get back to you."
Zuckerberg repeated much of what he said Tuesday about how the company is working to prevent interference in future elections, after it was found Russian-linked Facebook accounts helped spread misinformation across the platform in the 2016 presidential elections.
He said the company is deploying artificial intelligence tools to catch fake accounts that might spread false information, but admitted it wouldn't be possible to completely stop it.
"For as long as Russia has people who are employed, who are trying to perpetrate this kind of interference, it will be hard for us to guarantee that we're going to fully stop everything," Zuckerberg said.