Facebook has confirmed about 200 million phone numbers from members have been exposed in an online database.
The company said it was investigating who had compiled the database and left it online unprotected.
The database of telephone numbers and Facebook IDs was discovered on an unprotected web server and was not password protected.
It is not believed to have been compiled or put there by Facebook.
The database was taken offline after the news site TechCrunch reported the issue to the web hosting company.
In April 2018, Facebook switched off a feature that let people search for other users by typing in their phone number.