One thing Zuckerberg has highlighted during his testimony is an ad transparency tool currently being tested in Canada.

The tool lets any user see all of the adverts a page is running on Facebook, Instagram and Messenger, even if they are not part of the intended audience.

It's expected to be available in the US before the midterm elections in November.

Facebook is also creating a searchable archive of past political adverts.

Last October, Facebook in Canada announced an election integrity initiative in the run-up to the country's federal election in 2019.

The initiative was launched after Canada's electronic spy agency warned in a report about cyber threats to the country's democracy that "adversaries could use social media to spread lies and propaganda to a mass audience at a low cost".

Senator Dick Durbin opens with a straightforward question. "Mr Zuckerberg, would you be comfortable sharing the hotel you stayed in last night?"

After a long delay and a slight laugh, Zuckerberg says "Um, uh, no!"

The crowd laughs - and Durbin continues asking if he would share the names of people he has messaged this week.

That's another no from Zuckerberg.

"I think that might be what this is all about," Durbin says.

Image source, Getty Images

Senator Graham presses Zuckerberg: “What do we tell our constituents, given what’s happened here, why you should self-regulate?”

“Well, senator, my position is not that there should be not regulation…”

“You embrace regulation?”

“I think the real question, as the internet becomes more important in people’s lives, is what is the right regulation…”

“But do you as a company welcome regulation?”

“I think if it’s the right regulation, then yes.”

“You think the Europeans have it right?

“I think that they get... things right.”

More laughter in the room.

Senator Lindsey Graham starts by asking who Facebook's biggest competitor is.

"Senator we have a lot of competitors," Zuckerberg begins.

"Who's your biggest?" Graham interrupts.

"Do you want just one? I don't think I can give one. Can I name a bunch?" he responds.

Graham says: "Let me put it this way: If I buy a Ford and I don't like it I can buy a Chevy.

"If I'm upset with Facebook, what's the equivalent product that I can go sign up for?"

"Is there an alternative to Facebook in the private sector," he asks.

When Zuckerberg demurs, Graham interject: "You don't think you have a monopoly?"

"It certainly doesn't feel like that to me," Zuckerberg responds, noting that many Americans use eight social media platforms.

There's a ripple of laughter in the room.

Senator Patrick Leahy mentions a UN report that linked Facebook to inciting genocide in Myanmar.

Leahy mentions a specific post that called for the death of a Muslim journalist.

"That threat went straight through your detection system and it spread very quickly and it took attempt after attempt after attempt and the involvement of civil society groups to get you to remove it.

"Why couldn’t it be removed within 24 hours?"

“Senator, what’s happening in Myanmar is a terrible tragedy,” replies Zuckerberg.

“We all agree with that,” Leahy cuts him off.

“OK,” Zuckerberg says, looking taken aback by the interruption.

The senator continues: “But UN investigators have blamed you, blamed Facebook, for playing a role in that genocide. We all agree it’s terrible. How can you dedicate resources to make sure such hate speech is taken down within 24 hours?”

“Yes we’re working on this," says Zuckerberg, saying they are hiring more Burmese language staff.

Read more:UN investigators - Facebook has turned into a beast in Myanmar

Zuckerberg says he is willing to "roll out controls" regarding consent in the US, but refuses to answer whether he would support European-style regulations in the US.

The European Union regulators are famously tough on tech companies when it comes to data collection.

Many Republicans in this hearing so far have appeared reluctant to push for further government regulations on social media companies, such as those that are currently being pushed in Europe.

Zuckerberg says: "Regardless of whether we implement the exact same regulation - I would guess that it would be somewhat different as we have somewhat different sensibilities in the US as to other countries - we’re committed to rolling out the controls and the affirmative consent and the special controls around sensitive types of technology like face recognition that are required in GDPR [European Union's General Data Protection Regulation], we're doing that around the world."

Senator Maria Cantwell asks Zuckerberg a series of questions about Palantir, a data analysis firm whose chairman, Peter Thiel, is a Trump donor and on Facebook's board, and whether Facebook employees worked with Cambridge Analytica during the election.

Zuckerberg gives clipped responses, and ultimately says he will have to get back to senators on the details.

Zuckerberg says he has not personally been interviewed by the office of Special Counsel Robert Mueller, who is investigating suspected Russian interference in the 2016 election.

But he says that other members of Facebook have been interviewed.

"Have you or anyone at Facebook by the special counsel's office?" he's asked.

"Yes", he says, before adding that "our work with the special counsel is confidential and I want to make sure that in an open session I'm not revealing something that's confidential".

Zuckerberg's PR team will be relieved that the senators grilling him seem to be the butt of the many jokes.

He is asked by Senator Feinstein about preventing foreign actors interfering in US elections.

"This is one of my top priorities in 2018 is to get this right. One of my greatest regrets in running the company is that we were slow in identifying the Russian information operations in 2016.

"We expected to do a number of more traditional cyber-attacks, which we did identify… but we were slow in identifying the new information operations."

He says Facebook has developed new AI tools to weed out fake accounts, which are being deployed not just for November’s US midterm elections but for votes in Brazil, Mexico, Pakistan, Hungary and India.

Image source, AFP

"Nothing in life is free," says Senator Orinn Hatch, asking how Facebook makes money without charging users a fee.

"Senator we run ads," Zuckerberg says, then cracks a smile.

Is Zuck referencing a future paid version of Facebook?

He notes that artificial intelligence can already detect most Islamist extremist content, but other offensive posts are only reacted to after a user has complained to the company.

"Hate speech, I am optimistic, that over a five- to 10-year period we will have AI [artificial intelligence] tools that can get into some of the linguistic nuances of different types of content to be more accurate in flagging things for our systems.

"But today we're just not there on that.

"So a lot of this still reactive, people flag it to us, we have people look at it, we have policies to make it as not subjective as possible.

"But until we get more automated there’s a higher error rate than I am happy with."

Image source, AFP

Senator Thune references a Wired magazine article, external looking back about how Facebook has responded to previous scandals - and what hasn't changed.

Image source, Getty Images

Several Twitter users are having a laugh at the technical questions about social media being asked by the mainly elderly senators.

This classic 2012 Twitter post from Senate Judiciary Committee Chairman Chuck Grassley has also been resurfacing.

"You can turn off third-party information," Zuckerberg says, when asked about how to stop seeing ads for a product that a user is not interested in.

"You consider my personal identifiable data the company's data, not my data, is that it?" asks Senator Nelson.

"No, senator," Zuckerberg responds, before taking a drink of water.

"We have made a lot of mistakes in running the company. I think it's pretty much impossible to start a company in your dorm room and then grow it to be the scale that we’re at now without making some mistakes.

"And because our service is about helping people connect and information those mistakes have been different."

"We did take action," Zuckerberg insists, "we took down the app and we demanded that both the app developer and Cambridge Analytica delete and stop using any data that they had. They told us that they did this. In retrospect it was clearly a mistake to believe them and we should have followed up and did a full audit then. And that is not a mistake that we will make."

"I believe its important to tell people how their information is going to be used," Zuckerberg says.

"Every single time" a user shares information, they are asked what community (friends, family, etc) they want to share that with, he notes.

"Long privacy policies are very confusing", he says, adding that the longer the terms of agreement become, the fewer people will agree to use the service.

"I believe it's important to tell people exactly how the information that they share on Facebook is going to be used," Zuckerberg tells Senator Grassley.

The embattled company at the centre of the data scandal is live-tweeting its response to the hearing, denying it still has all the information from the breach.