"Given the scale and sophistication of the activity we've seen, we believe most people on Facebook could have had their public profile scraped in this way", the company said. Over half a million of the users whose personal data might have been compromised are from India.
Zuckerberg, in Wednesday's call, said he couldn't be sure.
"When you're building something like Facebook which is unprecedented in the world, there are things that you're going to mess up..."
"We don't share your information with advertisers".
On a question if he would testify in front of Parliamentarians in the House of Commons in the United Kingdom, the Facebook CEO said: "We announced today that I'm going to be testifying in front of Congress".
"I'm quite confident it will not be more than 87 million, it could well be less", he said.
Graphic on Facebook's advertising revenue.
More than 300,000 Australians may have had their personal information exposed in the Cambridge Analytica privacy scandal, as Facebook revealed the data leak has widened to 87 million users.
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During Zuckerberg's press conference, Cambridge Analytica tweeted it had only obtained data for 30 million individuals - not 87 million - from the app's creator, and again insisted it had deleted all records. This data included information from the users' personal profiles and their friends on the platform. Now Facebook is cracking down on all third-party apps to limit their reach, though it remains to be seen how well Facebook will implement those requirements and for how long.
Facebook has come under fire in recent weeks after it was disclosed that political consulting firm Cambridge Analytica, which worked for Donald Trump's presidential campaign, gained access to the personal data of 50 million Facebook users.
Facebook also said its new terms of service would provide clearer information on how data is collected and shared without giving the social network additional rights.
Earlier Wednesday, the House of Representatives' Energy and Commerce Committee announced what appeared to be the first congressional appearance by Zuckerberg since the scandal broke. When Facebook learned about the information being shared, it asked Cambridge Analytica to destroy the data.
"We started approving these permissions in 2014, but now we're tightening our review process - requiring these apps to agree to strict requirements before they can access this data", the CTO said.
He said that while "there are billions of people who love the service", there is also a potential for abuse and manipulation.
It's important to show people in black and white how our products work - it's one of the ways people can make informed decisions about their privacy. Facebook said the request was overly broad, hence its decision to deny access.