Facebook has said that the personal information of up to 87 million users, mostly in the United States, may have been improperly shared with political consultancy Cambridge Analytica, up from a previous estimate of more than 50 million.

Chief Executive Mark Zuckerberg said in a conference call with reporters that Facebook had not seen "any meaningful impact" on usage or ad sales since the scandal, although he added, "it's not good" if people are unhappy with the company.

Mr Zuckerberg told reporters that he accepted blame for the data leak, which has angered users, advertisers and politicians, while also saying he was still the right person to head the company he founded.

"When you're building something like Facebook that is unprecedented in the world, there are going to be things that you mess up," he said, adding that the important thing was to learn from mistakes.

He said he was not aware of any discussions on the Facebook board about him stepping down, although directors would face a challenge if they wanted to oust him because Mr Zuckerberg is the controlling shareholder.

He said he had not fired anyone over the scandal and did not plan to.

"I'm not looking to throw anyone else under the bus for mistakes that we made here," he said.

Facebook said it was taking steps to restrict the personal data available to third-party app developers. 

The world's largest social media company has been hammered by investors and faces anger from users, advertisers and politicians after a series of scandals about fake news stories, election-meddling and privacy.

Last month, Facebook acknowledged that personal information about millions of users wrongly ended up in the hands of Cambridge Analytica.

The previous estimate of more than 50 million Facebook users affected by the data leak came from two newspapers, the New York Times and London's Observer, based on their investigations of Cambridge Analytica.

Mr Schroepfer did not provide details of how Facebook came to determine its higher estimate, but he said Facebook would tell people if their information may have been improperly shared with Cambridge Analytica.

The British-based consultancy has denied wrongdoing. It says it engaged a university professor "in good faith" to collect Facebook data in a manner similar to how other third-party app developers have harvested personal information.

Facebook has said it planned to revise the written policies that people agree to when they use the social network, adding language about the protection of personal data as it prepares to comply with a strict new European law.

The company said it was publishing draft revisions of two documents that apply worldwide, its terms of service and its data policy, and was seeking feedback on them in advance of making them final.

The updates do not ask for new rights to collect, use or share data and will not affect the privacy settings that people have made on their Facebook accounts, Rob Sherman, Facebook's deputy chief privacy officer, said in a phone interview.

It comes as Mr Zuckerberg prepares to testify before the US Congress next week over the hijacking of users' personal data by Cambridge Analytica.

The hearing, set for 10am on 11 April before the House Energy and Commerce Committee, aims to "shed light on critical consumer data privacy issues and help all Americans better understand what happens to their personal information online," committee chairman Greg Walden and ranking Democrat Frank Pallone said in a statement.

"We appreciate Mr Zuckerberg's willingness to testify before the committee, and we look forward to him answering our questions."

Mr Zuckerberg also has been invited to appear before the Senate Judiciary Committee on 10 April, alongside Google chief Sundar Pichai and Twitter head Jack Dorsey.

His participation is not yet confirmed but Senator Dianne Feinstein told the San Francisco Chronicle Mr Zuckerberg had agreed to attend that hearing.