SAN FRANCISCO (REUTERS, AFP) - Facebook Inc Chief Executive Mark Zuckerberg apologised on Wednesday (March 21) for mistakes his company made in how it handled data belonging to 50 million of its users and promised tougher steps to restrict developers' access to such information.

The world's largest social media network is facing growing government scrutiny in Europe and the United States about a whistleblower's allegations that London-based political consultancy Cambridge Analytica improperly accessed user information to build profiles on American voters which were later used to help elect US President Donald Trump in 2016.

Zuckerberg, in his first public comments since the scandal erupted at the weekend, said in a post on Facebook that the company "made mistakes, there's more to do, and we need to step up and do it".

He said the firm had "a responsibility to protect your data".

"If we can't then we don't deserve to serve you," he said, adding: "I've been working to understand exactly what happened and how to make sure this doesn't happen again."

Zuckerberg later told CNN: "This was a major breach of trust. I'm really sorry this happened. We have a basic responsibility to protect people's data."

He told CNN that Facebook was committed to stopping interference in the US midterm election in November and elections in India and Brazil.

Zuckerberg said he was open to additional government regulation and happy to testify before the US Congress if he was the right person.

"I'm not sure we shouldn't be regulated," he said. "I actually think the question is more what is the right regulation rather than yes or no, should it be regulated? ... People should know who is buying the ads that they see on Facebook."

His plans did not represent a big reduction of advertisers' ability to use Facebook data, which is the company's lifeblood.

Zuckerberg also told the website Recode that he was open to testifying before the US Congress and that fixes to protect users' data would cost "many millions of dollars".

Facebook shares pared gains on Wednesday after Zuckerberg's post, closing up 0.7 per cent. The company has lost more than US$45 billion of its stock market value over the past three days on investor fears that any failure by big tech firms to protect personal data could deter advertisers and users and invite tougher regulation.

Zuckerberg told the New York Times in an interview published on Wednesday that he had not seen a "meaningful number of people" deleting their accounts over the scandal.

Cambridge Analytica obtained data on millions of people from an app developed by researcher Aleksandr Kogan. The app was downloaded by thousands of Facebook users but it also scooped up information from their friends.

"This was a breach of trust between Kogan, Cambridge Analytica and Facebook," Zuckerberg wrote.

"But it was also a breach of trust between Facebook and the people who share their data with us and expect us to protect it."

"We need to fix that."

Zuckerberg said some changes were made by Facebook in 2014 that addressed "abusive apps", and that "these actions would prevent any app like (the one linked to Cambridge Analytica) from being able to access so much data today".

He did not elaborate on what mistakes were made by the company, but he said the social network plans to conduct an investigation of apps on its platform, restrict developer access to data, and give members a tool that lets them more easily disable access to their Facebook data.

He did not explicitly apologise for the improper use of data, and his plans did not represent a big reduction of advertisers' ability to use Facebook data, which is the company's lifeblood.

Facebook shares pared gains on Wednesday after Zuckerberg's post, closing up 0.7 per cent. The company has lost more than US$45 billion (S$59.3 billion) of its stock market value over the past three days on investor fears that any failure by big tech firms to protect personal data could deter advertisers and users and invite tougher regulation.

Facebook representatives, including deputy chief privacy officer Rob Sherman, met US congressional staff for nearly two hours on Wednesday and planned to continue meetings on Capitol Hill on Thursday. Facebook was unable to answer many questions, two aides who attended the briefing said.

The whistleblower who launched the scandal, Christopher Wylie, formerly of Cambridge Analytica, said in a tweet that he had accepted invitations to testify before US and British lawmakers.

The German government said Facebook must explain whether the personal data of the country's 30 million users was protected from unlawful use by third parties, according to a report in the Funke group of German regional newspapers.

'SCAPEGOAT'

On Tuesday, the board of Cambridge Analytica suspended its chief executive Alexander Nix, who was caught in a secret recording boasting that his company played a decisive role in Trump's victory.

But psychologist Kogan, who provided the data on Facebook users, disputed that on Wednesday.

"I think what Cambridge Analytica has tried to sell is magic, and they've made claims that this is incredibly accurate and it tells you everything there is to tell about you. But I think the reality is it's not that," Kogan, an academic at Cambridge University, told the BBC in an interview broadcast on Wednesday.

Kogan, who gathered the data by running a survey app on Facebook, also said that he was being made a scapegoat by Facebook and Cambridge Analytica. Both companies have blamed Kogan for alleged data misuse.

Only 300,000 Facebook users responded to Kogan's quiz, but that also gave the researcher access to those people's Facebook friends, who had not agreed to share information, producing details on 50 million users.

Facebook has said it subsequently made changes that prevent people from sharing data about friends, and maintains that no data breach occurred because the original users gave permission. Critics say that it essentially was a breach because data of unsuspecting friends was taken.

Facebook banned Cambridge Analytica from using any of Facebook's services on Friday.

Zuckerberg said the company "will restrict developers' data access even further to prevent other kinds of abuse" and that the company is working with regulators as they investigate what happened.

Many analysts have now raised concerns that the incident will have a negative impact on user engagement with Facebook, potentially reducing its clout with advertisers. Three Wall Street brokerages cut their price targets.

"Investors now have to consider whether or not the company will conclude that it has grown in a manner that has proven to be untenable or whether it needs to significantly improve how it is managed," said Pivotal Research Group analyst Brian Wieser.

Facebook shares are down more than 8 per cent since Friday. The company has risen more than 550 per cent in value in the past five years.