Data held by Facebook on 87 million of its users may have been improperly shared with Cambridge Analytica, the company has said.
The number is 37 million higher than previous estimates in the information leak scandal.
In a blogpost, chief technology officer, Mike Schroepfer, said: "In total, we believe the Facebook information of up to 87 million people - mostly in the US - may have been improperly shared with Cambridge Analytica."
The social network says most of those affected were in the US - and has announced new privacy tools to protect users' data.
At the start of a scheduled conference call with members of the media about privacy on the platform, Facebook founder and CEO Mark Zuckerberg admitted the company didn't do enough to protect users' information, saying: "We didn't take a broad enough view of what our responsibility is. That was a huge mistake. It was my mistake."
On his previous dismissal of claims that false information influenced the US election, Mr Zuckerberg told reporters: "I clearly made a mistake by dismissing fake news as crazy."
"It was too flippant, and I should never have described it as crazy," he added.
Mr Zuckerberg said he and the company now took the issue more seriously and pointed to the recent removal of numerous accounts by the Russian Internet Research Agency, which he said showed their progress against disinformation.
But he added that tackling issues of security online was a "never-ending" arms race.
Asked if the Facebook board had discussed whether he should step down as chairman, Mr Zuckerberg replied only: "Not that I'm aware of."
When asked subsequently if he believed he was the right person to lead the company forward, Mr Zuckerberg said: "Yes."
"I think life is about learning from your mistakes and working out what you need to do to move forward," he said.
He added: "When you're building something like Facebook that is unprecedented in the world there are going to be things you mess up."
As Mr Zuckerberg spoke, Cambridge Analytica tweeted that the company only licensed data for "30 million individuals, not 87 million".
"We did not receive more than 30 million records from research company GSR," it said.
Cambridge Analytica licensed data from GSR for 30 million individuals, not 87 million. We did not receive more than 30 million records from research company GSR.— Cambridge Analytica (@CamAnalytica) April 4, 2018
The company added: "When Facebook contacted us to let us know the data had been improperly obtained, we immediately deleted the raw data from our file server, and began the process of searching for and removing any of its derivatives in our system."
When asked about this during the conference call, Mr Zuckerberg said that 87 million number was "the maximum number" of people whose data could have been accessed.
"We don't actually know how many people's information [Professor Aleksandr] Kogan got, we don't know what he sold and we don't know what they have on their system," Mr Zuckerberg said.
When asked if he had considered taking legal action against Cambridge Analytica following the scandal, Mr Zuckerberg said he would first allow the UK's Information Commissioners Office to investigate, but added: "We will take legal action if we need to do that to protect people's information."