Facebook says it now believes up to 87 million people's data was improperly shared with the political consultancy Cambridge Analytica.
The overall figure had been previously quoted as being 50 million by the whistleblower Christopher Wylie.
The details were revealed in a blog by the tech firm's chief technology officer, Mike Schroepfer.
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.
It was published several hours after the US House Commerce Committee announced that Facebook's founder, Mark Zuckerberg, would testify before it on 11 April.
The tech firm has faced intense criticism after it emerged that it had known for years that Cambridge Analytica had harvested data from millions of its users, but had relied on the London-based firm to self-certify that it had deleted the information.
Channel 4 News has since reported that at least some of the data in question is still in circulation despite Cambridge Analytica insisting it had destroyed the material.
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 lawmakers 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.
Shares in Facebook were down 1.4 percent on Wednesday to $US153.90. They are down more than 16 percent since the Cambridge Analytica scandal broke.
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.
Cambridge Analytica is accused of selling the personal information of Facebook users to influence the US presidential election and the Brexit referendum.
- Reuters / BBC