'High noon' on Capitol Hillpublished at 19:24 British Summer Time 10 April 2018
Democratic Senator Richard Blumenthal tells the BBC that Zuckerberg faces a "moment of reckoning".
'Moment of reckoning' for Zuckerberg
Facebook chief Mark Zuckerberg has told the US Senate that privacy lapses are his responsibility
"It's clear now that we didn’t do enough to prevent these tools from being used for harm," he says
Cambridge Analytica, a political consultancy linked to the Trump campaign, scraped data from 87m Facebook users
Zuckerberg, 33, is worth about $64bn, and is one of the world's youngest billionaires
Max Matza and Taylor Kate Brown
Democratic Senator Richard Blumenthal tells the BBC that Zuckerberg faces a "moment of reckoning".
'Moment of reckoning' for Zuckerberg
It's not so straight-forward.
Facebook denies that they sell users' data. Rather, they argue that they simply allow third parties to access that data for marketing purposes.
In a phone call with journalists earlier this month, Zuckerberg said: "For some reason, we haven’t been able to kick this notion, for years, that people think that we sell data to advertisers."
"We don’t."
But critics say that accepting payments from data analytics companies (such as Cambridge Analytica) for access to data is the same.
Zuckerberg will likely to be asked to clarify his assurance that Facebook will never sell data, which he made to the BBC in 2009.
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Facebook data: How it was used by Cambridge Analytica
BBC technology correspondent Dave Lee has followed Mark Zuckerberg's apology tour from Silicon Valley to Washington.
Watch his Twitter account as he speaks to some of the lawmakers who are expected to grill Zuckerberg today.
"Never have we had such unfiltered access to a man who is typically wrapped in cotton wool by his PR team and deputies," says Lee in his preview piece.
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Mark Zuckerberg is about to face one of the biggest tests of his meteoric career since he created Facebook in his Harvard dorm 14 years ago. The famously private tech titan, who has never testified in a congressional hearing, is about to undergo a gruelling, two-day inquisition on Capitol Hill.
On Tuesday, he faces two committees, on which 40 senators sit, in what may be a marathon hearing. The 33-year-old, sometimes depicted as robotic, has hired outside consultants to help coach him, even holding mock sessions to ready him for lawmakers' questions. Zuckerberg visited senators in closed-door meetings on Monday, previewing the public apology he plans to give Congress.
The social network has been under siege since revelations surfaced that Cambridge Analytica, a data-mining firm affiliated with Donald Trump’s presidential campaign, harvested data from 87 million users. "It was my mistake, and I'm sorry," Zuckerberg will say. I started Facebook, I run it, and I’m responsible for what happens here."
Cardboard cut-outs of Mark Zuckerberg outside the US Capitol