Facebook officials met with Hamburg's Data Protection Authority on Thursday, but the two entities are still at loggerheads over how the social-networking site collects the information of unregistered users.

Another meeting is scheduled for 24 August, said Johannes Caspar, who heads the agency. The DPA is concerned about Facebook's address book synchronisation feature, which will upload the email addresses of people who are not Facebook users from a registered user's existing email address contact lists.

Facebook asks users if they want to send an invite to their friends to join the site, but even if the email isn't sent, the email addresses are still retained. The agency is also concerned that email invites that are sent may violate laws prohibiting certain kinds of direct mail.

The DPA contends that many citizens of the German state of Hamburg have complained in recent months that Facebook passed their contact information to third parties and stores information about their relationships in this way.

"We will have another meeting and discuss the things they [Facebook officials] told us today," Caspar said. "Maybe they will offer us a solution but it's too early to say something in this case."

If it is found to violate German data protection laws, Facebook could be fined by the DPA. Facebook officials could not be immediately reached for comment.