Currently, he says, Facebook is "getting to the bottom of what Cambridge Analytica did", and notifying users of the breach.

"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. I started Facebook when I was in college.

"We’ve come a long way since then. We now serve more than two billion people around the world, and every day, people use our services to stay connected with the people that matter to them most. I believe deeply in what we’re doing. And when we address these challenges, I know we’ll look back and view helping people connect and giving more people a voice as a positive force in the world."

Image source, Getty Images

Zuckerberg reads from his opening statement, chunks of which he has memorised: "It's clear now that we didn’t do enough to prevent these tools from being used for harm as well. That goes for fake news, foreign interference in elections, and hate speech, as well as developers and data privacy. We didn’t take a broad enough view of our responsibility, and that was a big mistake. It was my mistake, and I’m sorry. I started Facebook, I run it, and I’m responsible for what happens here."

This is "not the first time that Facebook has mishandled" data, notes Democratic Senator Bill Nelson, who adds that a third party "lied" to consumers and Facebook.

"Why didn't Facebok notify 87 million users that their personal information had been taken?" he asks, adding that the data was used for "unauthorised political purposes".

"I think you're genuine. I got that sense conversing with you" yesterday, the senator adds.

We're streaming Mark Zuckerberg's testimony in front of Congress on - yes, you guessed it, Facebook Live and sharing related news.

Judiciary committee chairman Chuck Grassley, a Republican, says: "In fact, President Obama's campaign developed an app utilising the same Facebook feature as Cambridge Analytica to capture the information of not just the app's users, but also millions of their friends."

"The effectiveness of these social media tactics can be debated.

"But their use of the past years across the political spectrum and their increased significance can not be ignored."

Image source, Getty Images

Zuckerberg sips water and looks straight ahead as the fourth senator finishes his remarks.

The market will be watching how Facebook executive Mark Zuckerberg fares in his Capitol Hill outing, but investors have already shown they’re worried.

Facebook’s value has fallen by more than $65bn since 16 March, when the most recent uproar about Cambridge Analytica started and the likelihood of tighter regulation increased.

Shares were trading at about $161 as the hearing started today, back where it was in July.

Image source, Getty Images

Democrat Dianne Feinstein begins her questioning by asking about Facebook's influence on the 2016 election.

Eighty-seven million people had their data taken by Cambridge Analytica, which was hired by the Trump campaign to target political advertising at American voters, she notes.

We can expect more questions today from Democrats about whether Facebook - through Russian trolls - unfairly affected the US election.

"The words you're about to tell us will indicate how strongly your industry will regulate and or reform the platforms that you control," the California Democrat tells Zuckerberg.

"Malicious actors" are "exploiting the very tools that you created to manipulate users information," says Senator Thune.

"For this model to persist, both sides of the bargain need to know the stakes involved," he says, adding that many users do not understand the terms of their agreement with the social media company.

Mr Thune goes on to describe the story of Facebook's creation as the perfect "American dream".

Zuckerberg is slightly nodding and occasionally glancing downward as the senator calls for further changes.

Image source, Getty Images

Data harvesting is a multibillion dollar industry and the sobering truth is that you many never know just how much data companies hold about you, or how to delete it.

"Thousands of companies are in the business of harvesting your data and tracking your online behaviour," says Frederike Kaltheuner, data programme lead for lobby group Privacy International.

"It's a global business. And not just online, but offline, too, via loyalty cards and wi-fi tracking of your mobile. It's almost impossible to know what's happening to your data."

Read more about how companies gather and utilise your personal data.

Congressman John Thune quotes Zuckerberg, saying that the release of data to Cambridge Analytica was a "breach of trust".

He says: "One reason so many people are worried about this incident is what it says about how Facebook works." He calls the data-scraping "disturbing".

Read more: Cambridge Analytica: The story so far

Image source, Getty Images

Dozens of cameras are scrambling to catch all the best angles of the 33-year-old tech billionaire as he enters the hearing room.

The normally media shy mogul will be taking questions after four senators give their introductory remarks.

He posed for photos for over a minute, wearing a suit and blue tie rather than his T-shirt and jeans.

Image source, Getty Images

Senator John Thune, on his way to one of two hearings today, tells the BBC that Zuckerberg's credibility is on the line.

He tells our producer at Capitol Hill that he wants to know what "they will do proactively to stop some of the harmful conduct that has happened on their platform".

Image source, Reuters

Facebook has confirmed a small number of Facebook users involved in the Cambridge Analytica breach gave access to their Facebook private messages.

About 1,500 users explicitly gave the app access to their inbox, but users who exchanged messages with those 1,500 would also be affected.

Carole Cadwalladr - the Observer journalist whose investigation helped plunge Facebook into the current crisis - was among the first to pick up on the implication.

Read more about who and what was affected.

Image source, Getty Images

Zuckerberg has just posted on his Facebook page, natch.

"In an hour I'm going to testify in front of the Senate about how Facebook needs to take a broader view of our responsibility - not just to build tools, but to make sure those tools are used for good," he writes.

"I will do everything I can to make Facebook a place where everyone can stay closer with the people they care about, and to make sure it's a positive force in the world."

Image source, Getty Images

A Politico piece published yesterday, external envisions a world 15 years from now, in which Facebook has successfully moved past the scandals that it faces today.

"Facebook’s perverse business model amplified paranoia, cynicism and bullying. Yet Zuckerberg defended it as if it were the crown jewel," writes Jaron Lanier, the author of Ten Arguments for Deleting Your Social Media Accounts Right Now, external.

He argues for a company that is owned by users, who pay a fee to use the platform.

"Suddenly, the users would also be the customers. Suddenly, Facebook would be making money without the creepy incentives of a manipulation-based economy."