Facebook appears to have fixed the problem with their site which meant it was sharing members' birthdates even if they had set their privacy settings to hide the information from everyone.
In tests I conducted this morning it was no longer possible to view other people's dates of birth unless they had given explicit permission to share the information. Of course, why anyone would want to publish their birthday on Facebook is beyond me (shouldn't your friends know when your birthday is?), but that's for another discussion.
Update 16 July 2008:
I've posted some details of the specifics of what was going on on Facebook's site on the SophosLabs blog. You can also view the video I made on Monday night demonstrating that it really was child's play to look up other people's private information.
(Enjoy this video? You can check out more on the SophosLabs YouTube channel and subscribe if you like)
You can read more about this issue in this advisory we have published on our website: "Facebook privacy breach exposed users' hidden dates of birth".