Zuckerberg repeatedly defended the company's privacy practices, saying that users have control over their own data and decide what to share. But when asked if his data had been improperly used he replied: "Yes." He gave no further details.

Once again wearing a dark suit instead of his usual grey T-shirt, the hearing was Zuckerberg's second in two days. On Tuesday, he took questions for nearly five hours without making any further promises to support new legislation or change how the social network does business.

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.

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The latest estimate of affected users is up to 87 million in countries including Australia.

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 Moscow denies.

Facebook shares turned positive after earlier falls on Wednesday. They posted their biggest daily gain in nearly two years on Tuesday as 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, Zuckerberg said on Wednesday, but he again steered away from any specifics.

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Some lawmakers grew frustrated at their limit of four minutes each to press 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 congresswoman Marsha Blackburn said at one point. She cut Zuckerberg off a number of times.

Bobby Rush, an Illinois Democratic congressman, was in the process of asking 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.

Reuters