The social media giant has revised its own estimation of how many people's data was accessed.
Facebook has said that 29 million people's personal data was accessed by hackers, 20 million fewer than was originally feared.
The social media company said the FBI asked them not to publicly reveal the source of the attack, but it has published a link allowing users to see if they are affected.
Hackers accessed sensitive information like relationship statuses, locations, recent searches and birthdates using access tokens - which permit easier access across devices.
Guy Rosen, Facebook's head of product management said in a statement: "We have not ruled out the possibility of smaller-scale attacks, which we’re continuing to investigate Of the 50 million people whose access tokens we believed were affected, about 30 million actually had their tokens stolen.
"As we look for other ways the people behind this attack used Facebook, as well as the possibility of smaller-scale attacks, we’ll continue to co-operate with the FBI, the US Federal Trade Commission, Irish Data Protection Commission, and other authorities."
The European Commission tweeted: "Today’s update from Facebook is significant now that it is confirmed that the data of millions of users was taken by the perpetrators of the attack.
"[The] investigation into the breach and Facebook’s compliance with its obligations under GDPR continues."
