Facebook will investigate "tens of thousands" of apps to discover if any other companies have accessed data in a similar way to Cambridge Analytica (CA), Mark Zuckerberg has told US politicians.
The social network says it is in the process of letting up to 87 million users know that their information may have been accessed by CA, and in a packed room on Capitol Hill, Mr Zuckerberg repeated his admission that the company "didn't do enough" to stop its tools "being used for harm".
Speaking to the Senate Judiciary and Commerce Committees, the tech entrepreneur apologised that Facebook had not taken a "broad enough view" of its responsibility for people's public information.
"It was my mistake, and I'm sorry. I started Facebook, I run it, and I'm responsible for what happens here," he said, in words from a prepared statement.
Mr Zuckerberg said its audit of third-party apps would highlight any misuse of personal information, and said the company would alert users instantly if it "found anything suspicious".
When asked why the company did not immediately alert the 87 million users whose data may have been accessed by Cambridge Analytica (CA) when first told about the "improper" usage in 2015, Mr Zuckerberg said Facebook considered it a "closed case" after CA said it had deleted it.
"In retrospect it was clearly a mistake to believe them," he said.
Culture Secretary Matt Hancock will meet Facebook representatives in London on Wednesday.
Facebook and Cambridge Analytica face multiple lawsuits over alleged misuse of personal information with at least five law firms in the UK and US investigating claims for compensation.
On the subject of fake news, Mr Zuckerberg said "one of my greatest regrets in running the company" was its slowness at uncovering and acting against disinformation campaigns by Russian trolls during the US election.
He said the Russian campaign of disinformation had been discovered "right around the time" of the US presidential election, and said the company had developed "new AI tools" to identify fake accounts responsible.
But he said: "They are going to keep getting better at this and we need to invest in keeping on getting better at this too."
After two-and-a-half hours of the hearings, Facebook shares were up around 4.5% on the start of the day, in an apparent signal of investors' view on Mr Zuckerberg's performance.