Facebook also published a website where users can go to check if their accounts where affected by the breach, and if so, to what degree their information was exposed.
The company said the breach is under investigation by the FBI, which asked Facebook "not to discuss who may be behind this attack."
"We are still looking at other ways the people behind these attacks may have used Facebook, and we haven't ruled out the possibility of smaller scale, low-level access attempts," said Guy Rosen, Facebook vice president of product management, adding that the company had also notified the U.S. Federal Trade Commission and the Irish Data Protection Commission.
"People's privacy and security are incredibly important, and we are sorry this happened," Rosen said.
The company said the attack began on Sept. 14 and was not detected until Sept. 25. Within two days, the company fixed its vulnerabilities, stopped the attack and reset the access tokens for impacted users, Rosen said. Those impacted users will receive a note from Facebook on the service in the coming days notifying them of the attack, Rosen said.
Facebook discovered and disclosed the security breach in late September, saying at the time that the issue affected 50 million accounts, with an additional 40 million deemed as "at-risk." That number was reduced to 30 million, according to a blog post published by the company.
The company has been dealing with several issues concerning the health of its service throughout 2018. Facebook on Thursday, for example, disclosed its decision to remove 559 Pages and 251 accounts that it claimed broke the company's spam policies.
Shares of Facebook, which were already down slightly before the company's announcement, fell to a day low of $151.30 per share after Friday's announcement.