Facebook Chief Executive Mark Zuckerberg has told US politicians that he was among around 87 million users whose data may have been improperly shared with political consultancy firm Cambridge Analytica.

However, he dismissed Congress members' suggestions that Facebook users do not have enough control of their data in the wake of the privacy scandal at the world's largest social media network.

"Every time that someone chooses to share something on Facebook ... there is a control. Right there. Not buried in the settings somewhere but right there," the 33-year-old told the US House of Representatives Energy and Commerce Committee.

Mr Zuckerberg faced questions and concerns from Congress members about what it was doing to give users more flexibility to opt in to sharing their personal data with Facebook or third parties.

"How can consumers have control over their data when Facebook does not have control over the data?" asked Representative Frank Pallone of New Jersey, the ranking Democrat on the Energy and Commerce committee, at the beginning of the hearing.

Mr Zuckerberg repeatedly defended the company's privacy practices, saying that users have control over their own data and decide what to share.

When asked if his data had been improperly used he replied: "Yes." He gave no further details.

The hearing was Mr Zuckerberg's second in two days. Yesterday he took questions for nearly five hours in a Senate hearing without making any further promises to support new legislation or change how the social network does business.

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What is Cambridge Analytica? 

Facebook has been consumed by turmoil for nearly a month, since it came to light that millions of users' personal information was wrongly harvested from the website by Cambridge Analytica, a political consultancy that has counted US President Donald Trump's election campaign among its clients.

The latest estimate of affected users is up to 87 million.

Patience with the social network had already worn thin among users, advertisers and investors after the company said last year that Russia used Facebook for years to try to sway US politics, an allegation that Russia denies.

Facebook shares turned positive after earlier falls this morning.

They posted their biggest daily gain in nearly two years yesterday as Mr Zuckerberg managed to deter any specific discussion about new regulations that might hamper Facebook's ability to sell ads tailored to users' profiles.

"It is inevitable that there will need to be some regulation" of internet firms, Mr Zuckerberg said, but he again steered away from any specifics.

Some polticians grew frustrated at their limit of four minutes each to press Mr Zuckerberg on specifics, and chastised the billionaire at times for offering up rehearsed platitudes about valuing user privacy.

"I can't let you filibuster right now," Republican Representative Marsha Blackburn said at one point. She cut Mr Zuckerberg off a number of times.

Bobby Rush, an Illinois Democratic congressman, was in the process of asking Mr Zuckerberg when he learned that Facebook allowed advertisers to prevent ads from being shown to certain minority groups, a possible violation of civil rights laws. He was cut off.