Facebook has apologised for the way it rolled-out a new system that recognises users' faces.
The social network said that it should have done more to notify members about the global launch.
Its Tag Suggestions feature scans photos and automatically picks out existing friends.
Although users have the option to switch it off, some complained that they were not explicitly asked if they wanted it activated.
Facebook said that the system was intended to speed up the process of assigning a name to a picture, known as tagging.
It was introduced in the US in December 2010 but has only now been launched globally.
Graham Cluley, senior consultant with security firm Sophos, said that users' annoyance was less about the product's purpose than the manner in which it was made live.
"Once again Facebook seems to be sharing personal information by default," said Mr Cluley.
"Many people feel distinctly uncomfortable about a site like Facebook learning what they look like and using that information without their permission