The hearing is still rolling on, and Zuckerberg has barely broken a sweat after several hours of grilling.

After a nervy start, he almost seems to be enjoying himself.

To recap, he apologised to US senators for allowing several other firms to misuse his platform's personal data for political ends. He said it was his personal mistake.

On the subject of fake news, a contrite Zuckerberg said "one of my greatest regrets in running the company" was its slowness at uncovering and acting against disinformation campaigns by Russian trolls during the US election.

He said Facebook will investigate "tens of thousands" of apps to discover if any other companies have accessed data in a similar way to Cambridge Analytica.

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Zuckerberg is shrewd enough not to appear big headed.

Alaska Senator Dan Sullivan tees one up for him about his achievement in creating the world's biggest social network.

"Guy in a dorm room... could only happen in America, right?"

Zuckerberg: "There are some strong Chinese internet companies."

Sullivan interrupts, and tells him it's a softball question: "The answer is yes," the senator laughs.

"Is Facebook too powerful?" a senator asks. "Are you too powerful? Do you think you're too powerful?"

Zuckerberg says that of the more than two billion Facebook users on Earth, many are outside the US.

"That's something that Americans should be proud of," the chief executive says.

Senator Mazie Hirono asks Zuckerberg when Facebook goes to law enforcement.

When there's immediate harm, he says.

Zuckerberg says Facebook would decline to provide information to Immigration and Customs Enforcement in deportation investigations.

"We're going to push back aggressively," he said.

Some people on Twitter think Zuckerberg mentions Facebook's Harvard dorm origin story too much...

"You're a dad," one senator notes, before asking if social media is addictive for teenagers.

"This is certainly something that I think any parent thinks about, how much do you want your kids using technology," Zuckerberg says.

"I view our responsibility as not just building services that people like, but building services that are good for people and good for society as well."

"Like any tool, there are good and bad uses of it," he adds.

"If you're interacting [with friends], that is associated with the long-term measures of well-being," he says.

But he adds that those who "passively view content" may feel negative effects.

The senator asks if Facebook studies "dopamine feedback loops" to keep people from leaving the platform.

Zuckerberg directly denies that Facebook ever consults such experts.

Zuckerberg says he would support "in principle" US legislation to require user permission before their data is used. He tells Senator Ed Markey that details of the legislation would matter and he looks forward to further talks.

Delaware Senator Chris Coons says there's a tension between Zuckerberg's "bold promises" on privacy and the reality that Facebook is a "for-profit" entity that made $40bn last year.

"There's lots of examples" where ad targeting becomes extremely effective in violating laws - including cases of discriminatory housing ads - and targeting people who would be most harmed, Coons says, like gambling ads for those with gambling addictions, liquor ads for alcoholics.

Facebook needs to enforce their own policies, Coons says.

So how is Zuckerberg's effort going to dispel the perception of him as a robot?

Erm...

Zuckerberg is told “people have no earthly idea of what they’re signing up for” in Facebook's terms of service.

Democratic Senator Brian Schatz: "If I'm emailing within WhatsApp [which Facebook owns] about Black Panther [the movie], do I get a Black Panther banner ad?"

Zuckerberg: "Facebook systems do not see the content of messages being transmitted over WhatsApp."

Senator: "Yeah, I know. But that’s not what I’m asking. I’m asking whether these systems talk to each other without a human being touching it?"

Zuckerberg looks perturbed as he responds: "Senator, I think the answer to your specific question is if you message someone about Black Panther on WhatsApp, it would not inform any ads."

Zuckerberg has told the congressional panel that "terms of service are what they are", adding that users do not have the option to amend what they agree to.

He also says it would be impossible for the company to know if foreign citizens or governments were creating shell companies to purchase adverts on Facebook without being publicly identified.

On Twitter, several users have pointed out that it would be very easy for a person or government - or "malicious actor", as senators have called them - to create a shell corporation to hide their adverts' intent.

US lawmakers are currently considering regulations that would require adverts to come with a label, saying who had payed for them.

Senator Ted Cruz: "Does Facebook consider itself to be a neutral public forum?"

Zuckerberg: "We consider ourselves to be a platform for all ideas."

Here's that deliciously awkward moment from earlier in the hearing.

Senator Richard Blumenthal says: "Your business model is to monetise user information to maximise profit over privacy.

"And unless there are specific rules and requirements enforced by an outside agency I have no assurance that these kinds of vague commitments are going to produce action.”

The Connecticut Democrat says companies ought to be required to provide users with clear and plain information about how their data will be used.

"Senator, I do generally agree with what you're saying," says Zuckerberg.

"Would you agree to an opt-in, as opposed to an opt-out?"

Zuckerberg noncommitally responds: "Senator, I think that that certainly makes sense to discuss and I think the details around this matter a lot."

In one of the more testy exchanges, Texas Republican Ted Cruz demands to know how many Facebook employees have backed Republican candidates in the past.

"Of those 15 to 20,000 people engaged in content review, how many - if any - have ever supported financially or politically a Republican candidate for office?"

"Senator, I do not know that," Zuckerberg responds.

Facebook has previously been accused of a pro-liberal bias.

PS: Cruz's 2016 presidential campaign used Cambridge Analytica.

Mark Zuckerberg says he will get back to Cantwell about whether Facebook employees worked with Cambridge Analytica.

New York Times political reporter Nick Confessore says he has an answer:

Markets have liked what Mark Zuckerberg has to say.

Facebook shares jumped by 4.5% to close at $165.04, with most of the gains occurring in late-afternoon trade.

That was the biggest one-day percentage gain since April 2016, according to Reuters news agency.

However, the share price remains about 15% lower than the peak it hit in February.

The committee chairman offers to take a quick break, but Zuckerberg, who has been drinking a lot of water during this hearing, said he can keep going.

"We can do a few more," he suggests as the testimony passes its second hour.

"Maybe, 15 minutes. Does that work?" he said to laughter in the room.

The senators seem happy to keep the question and answer session going.

"There's a very common misconception about Facebook, that we sell data to advertisers," says Zuckerberg. "And we we do not sell data to advertisers."

The senator interjects: "Well, you clearly rent it."

"Um, what we allow is for advertisers to tell us who they want to reach and then we do the placement."

Facebook, he says, just "shows the ads to the right people without that data ever changing hands and going to the adverstisers."

"That's a very fundamental part of how our model works," he adds.