Facebook CEO Mark Zuckerberg has revealed during a second day of sparring with US politicians over privacy concerns that he was among the 87 million users whose data was improperly shared.

Key points: Mark Zuckerberg faces US House Energy and Commerce Committee

Mark Zuckerberg faces US House Energy and Commerce Committee He defends Facebook's privacy practices, saying users have control over their own data

He defends Facebook's privacy practices, saying users have control over their own data Mr Zuckerberg says Facebook does collect information on people not signed up

The admission that even the tech-savvy Facebook founder was unable to protect his own data underscored the problem Facebook has in persuading sceptical politicians that users can easily safeguard their own information.

"Every time that someone chooses to share something on Facebook … there is a control," he said.

"Right there. Not buried in the settings somewhere but right there."

Yet, when asked if his data had been improperly shared with Cambridge Analytica, he replied: "Yes." He gave no further details.

The 33-year-old internet magnate faced questions and concerns from members of the US House of Representatives Energy and Commerce Committee, who asked what Facebook was doing to give users more flexibility to opt in to sharing their personal data with the company or third parties.

"How can consumers have control over their data when Facebook does not have control over the data?" asked Congressman Frank Pallone of New Jersey 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.

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The Facebook boss said he was not familiar with so-called "shadow profiles", which media reports have described as collections of data about users that they have no knowledge of or control over.

He also said Facebook does not collect information from users' verbal conversations through mobile devices' microphones.

However, in a series of questions on how people can remove data from Facebook, Mr Zuckerberg said Facebook does "collect data on people who are not signed up for Facebook for security purposes".

He had no response when asked how a person who is not a Facebook member can remove information without first signing up for the service.

'It is inevitable that there will need to be some regulation'

Sorry, this video has expired Mark Zuckerberg addresses Facebook privacy leak on first day of hearings (Photo: Reuters/Aaron P. Bernstein)

Mr Zuckerberg was on Capitol Hill for the second time in two days to answer questions about data privacy.

It comes in the wake of revelations last month 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.

The data was improperly shared with Cambridge Analytica by an academic who gathered data on users and their friends through a questionnaire app on Facebook.

Facebook has since shut off the ability of apps to gather such data, but Mr Zuckerberg said it would take "many months" to complete an audit of other apps to determine if they also improperly used data.

The House hearing came a day after a five-hour questioning by US senators, in which Mr Zuckerberg made no further promises to support new legislation or change how the social network does business.

Facebook shares in the US were up 1.5 per cent on Wednesday (local time) after dips earlier in the day.

The latest estimate of users affected by the scandal is up to 87 million. ( AP: Pablo Martinez Monsivais )

They posted their biggest daily gain in nearly two years on Tuesday 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," Mr Zuckerberg said, but steered away from any specifics.

Some politicians 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 Marsha Blackburn said at one point. She cut Mr Zuckerberg off a number of times.

Democrat Bobby Rush was in the process of asking Mr Zuckerberg a question 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.

"I am indeed wary that you are only acting now out of concern for your brand and are making changes that should have been made long ago," Democrat Paul Tonko said.

Reuters