(Updates with start of hearing)
WASHINGTON/SAN FRANCISCO, April 11 (Reuters) - Facebook Inc Chief Executive Mark Zuckerberg started a second day of testimony on Capitol Hill on Wednesday, facing more questions from lawmakers about data privacy at the world's largest social media network.
The 33-year-old internet magnate, once again wearing a dark suit instead of his usual gray T-shirt, appeared before the U.S. House of Representatives Energy and Commerce Committee, a day after he took questions for nearly five hours in a U.S. Senate hearing.
He navigated through the first hearing on Tuesday without making any further promises to support new legislation or change how the social network does business, foiling attempts by senators to pin him down.
Investors were impressed with his initial performance. Shares in Facebook posted their biggest daily gain in nearly two years on Tuesday, closing up 4.5 percent. They were down slightly in early trading on Wednesday.
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 U.S. 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 U.S. politics, an allegation Moscow denies.
Lawmakers have sought assurances that Facebook can effectively police itself, and few came away from Tuesday's hearing expressing confidence in the social network.
"I don't want to vote to have to regulate Facebook, but by God, I will," Republican Senator John Kennedy told Zuckerberg on Tuesday. "A lot of that depends on you."
Zuckerberg deflected requests to support specific legislation. Pressed repeatedly by Democratic Senator Ed Markey to endorse a proposed law that would require companies to get people's permission before sharing personal information, Zuckerberg agreed to further talks.
"In principle, I think that that makes sense, and the details matter, and I look forward to having our team work with you on fleshing that out," Zuckerberg said.
(Reporting by Dustin Volz in Washington and David Ingram in San Francisco Editing by Bill Rigby)