Facebook says hackers accessed data from 29 million accounts as part of the security breach disclosed two weeks ago.
The exact number had not been known before.
Originally Facebook said 50 million accounts could have been affected, but they did not know if they had been misused.
The hackers accessed names, email addresses or phone numbers from those 29 million accounts.
For 14 million of those accounts, hackers got even more data, such as home town, birth date, the last 10 places they checked into or 15 most recent searches.
One million accounts were affected but no information was gained.
The social media service plans to send messages to people whose accounts were hacked.
It says third-party apps and Facebook apps like WhatsApp and Instagram were unaffected by the breach.
Facebook says the FBI is investigating, but authorities asked them not to discuss who may be behind the attack.
That suggests Facebook may know or suspect who is actually behind the breach.