Video report by ITV News Reporter Ivor Bennett
Facebook boss Mark Zuckerberg admitted that he was one of the 87 million users whose data was sold to Cambridge Analytica and revealed his firm was exploring legal action against Cambridge University.
On the second day of the Congressional hearing, the CEO argued that advertisers and developers would never take priority over the company's users but was forced to admit that his data had also been sold during the scandal.
He also revealed that the company is looking into whether "something bad" was going on at Cambridge University, where one of the researchers involved in the scandal was based.
Here are some of the key points from the hearing so far:
On the second day of the hearing, 33-year-old Mark Zuckerberg said that his company is exploring whether to take action against Cambridge University, where a researcher involved in the data breach scandal was based.
An app, designed by Cambridge University researcher Aleksandr Kogan, is said to have collected Facebook users' details on behalf of the British wing of Cambridge Analytica.
"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," Mr Zuckerberg told the Congressional hearing.
"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."
"We have a responsibility to make sure what happened with Kogan and Cambridge Analytica doesn't happen again," Mr Zuckerberg added.
Mr Zuckerberg was forced to acknowledge that his own personal data had been breached alongside millions of other users in the Cambridge Analytica scandal.
When Representative Anna Eshoo asked whether he was "included in the data sold to the malicious third parties", the Facebook CEO replied: "Yes."
Representative Bobby Rush said Facebook orchestrates a "wholesale invasion and manipulation of [citizens'] right to privacy" similar to state surveillance of civil rights movements in the 1960s.
Mr Zuckerberg argued that Facebook users are able to choose and control what they share on the platform.
On the first day of the hearing, Senator Dick Durbin used Mr Zuckerberg's personal life to drive home a point about privacy.
Senator Durbin said: "Mr Zuckerberg, would you be comfortable sharing with us the name of the hotel you stayed in last night?"
After a short pause Mr Zuckerberg replied: "No."
"If you messaged anyone this week would you share with us the names of the people you have messaged?" Sen Durbin responded.
"Senator, no, I would probably not choose to do that publicly here," Mr Zuckerberg said.
Durbin continued: "I think that may be what this is all about: your right to privacy, the limits of your right to privacy, and how much you give away, in modern America, in the name of connecting people around the world."
The social media boss said that regulation of his industry was "inevitable".
Politicians in the US and UK are weighing up the possibility of legislation to regulate large technology companies like Facebook, Google and Twitter which are accused of failing to regulate themselves.
When asked earlier in the hearing if Facebook is "too powerful?" Mr Zuckerberg alluded to being open to regulation.
He said: "We need to have a conversation about the right regulation."
Mr Zuckerberg opened the second day of the hearing saying: "My top priority has always been our social mission of connecting people, building community and bringing the world closer together.
"Advertisers and developers will never take priority over that as long as I'm running Facebook."
On the first day of the hearing, Mr Zuckerberg took responsibility for the content on Facebook, a definite shift away from the platform's previous attitude to material posted on its site.
He said Facebook didn't do enough to prevent its tools from being used for harm.
This, he added, goes for fake news, foreign elections interference, hate speech and data privacy.
"It was my mistake, and I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here," he said.
'Arms race' with Russia
Mr Zuckerberg said one of his "greatest regrets" as Facebook's boss had been the company's slowness at combating "disinformation campaigns by Russian trolls" during the US election.
He said the campaign was discovered "right around the time" of the 2016 election, adding that new AI tools had since been developed to identify fake accounts.
"You know there are people in Russia whose job is to exploit our systems... so this is an arms race," he said.
"They are going to keep getting better at this and we need to invest in keeping on getting better at this too."
"The sheer volume of content on Facebook makes it so that no amount of people we can hire will be enough to review all of the content," Mr Zuckerberg said on Wednesday.
He stressed that the company needs to "rely on and build sophisticated AI tools that will helps us flag certain content".
He mentioned terrorist content as an area in which artificial intelligence allows Facebook to "take down 99% of al Qaida and Isis content" before it is reported by users.
Senator Lindsey Graham asked the Silicon Valley executive if Facebook had a monopoly.
He said: "If I have a Ford and it doesn't work I'd buy a Chevy. What's the equivalent for Facebook?
"...Do you have a monopoly?"
To which Mr Zuckerberg responded: "It doesn't feel like that to me."
In defence of the platform he said it provides a "number of different services" and that the average person uses eight different apps to communicate with friends.
When it comes to digital advertising however, Facebook is considered to be a duopoly with Google.
Facebook as a free service
When asked about the business model for Facebook, Mr Zuckerberg maintained that there would always be a version that is free to users.
Senator John Thune noted that people were getting a free service in exchange for giving up personal data.
"For this model to persist, both sides of the bargain need to know what’s involved," Thune said.
"I’m not convinced Facebook’s users have the information they need to make decisions."