Facebook says the personal data of up to 87 million users may have been improperly shared with a British political consultancy firm, 37 million more than previously thought.
Last month it was revealed that the users' information was obtained by the firm Cambridge Analytica, which worked for Donald Trump's presidential campaign.
Facebook chief executive, Mark Zuckerberg, says he is committed to restoring Facebook's reputation.
Facebook said it would inform users if their information had been funnelled to Cambridge Analytica. It said roughly 70 million of those users were in the United States.
And in seeking to reassure users that it was moving to safeguard their personal information, the company made an extraordinary disclosure: chief technology officer Mike Schroepfer said the majority of its users were vulnerable to abuse of a now-disabled feature allowing people to search for other users with phone numbers and email addresses.
