In his opening remarks Mr Zuckerberg said that Facebook was an "idealistic" company
Facebook's chief is answering US politicians' questions about the Cambridge Analytica data privacy scandal for a second day in Washington.
During a testy early exchange, Mark Zuckerberg declined to give a commitment to change all users' default privacy settings to collect the minimum amount of personal information.
"This is a complex issue," Mr Zuckerberg said.
"That's disappointing to me," responded Democratic congressman Frank Pallone.
Later, Mr Zuckerberg was asked whether he planned legal action against Aleksandr Kogan, the researcher who had gathered and sold users' data to the political consultancy Cambridge Analytica.
In his response, Mr Zuckerberg revealed he was also exploring options against the university Kogan was based at.
"What we found now is that there's a whole programme associated with Cambridge University where... there were a number of other researchers building similar apps," he said.
"So, we do need to understand whether there was something bad going on at Cambridge University overall that will require a stronger reaction from us."
Elsewhere in the hearing, Mr Zuckerberg acknowledged that he believed it was "inevitable" that the internet would need new regulations.
"My position is not that there should be no regulation, but I also think you have to be careful about what regulation you put in place," he added.
The 33-year-old also faced fresh accusations from Republican congressman Steve Scalise that the News Feed's algorithm was discriminating against conservative news and content in favour of liberal posts.
"There is absolutely no directive in any of the changes that we make to have a bias," responded Mr Zuckerberg.
"To the contrary our goal is to be a platform for all ideas."
- Zuckerberg: Facebook is in 'arms race' with Russia
- Facebook and Google 'must be paid for'
- Facebook: Data contained private messages
Exposed notes
Tuesday's session lasted five hours.
However, some of the questions that Mr Zuckerberg had expected were not asked.
Photographs of his notes captured by Associated Press revealed that he had prepared a fresh defence to Apple's criticism of his company's ad-driven business model.
"On data, we're similar. When you install an app on your iPhone, you give it access to some information, just like when you login with Facebook," the notes said.
"[There are] lots of stories about apps misusing Apple data, [but I have] never seen Apple notify people," his prepared notes added.
Other developments over the past day include:
- The European Commissioner for consumers and justice has told the Guardianshe may propose new regulations to tackle a "loss of trust" in Facebook, and would raise the matter with the tech firm's chief operating officer Sheryl Sandberg later this week
- Cambridge Analytica has sent letters to publishers including the BBC warning that it will treat any misleading or inaccurate reports about itself with the "utmost seriousness"
- The UK Culture Secretary Matt Hancock has met with Facebook officials in London
