A flaw in a beta version of Facebook made it possible to see member birth dates, even those set to hide this information. Birth dates are often used to confirm someone’s identity. By having a full name and birth date it’s possible to phone up companies and ask for more private information (this is called Identity Theft).
Facebook has already fixed the flaw. However it’s a good reminder that any private information you enter into a social network such as Facebook could some day be read by someone not meant to read it.
If something is important enough to be private then don’t enter it into someone else’s system without thinking through the potential consequences.
You can view a video of how this flaw works here.