Secondlife, a 3-D virtual world entirely built and owned by its residents discovered an exploit last saturday permitting *hackers* to have a peek in the database behind the site :

Detailed investigation over the last two days confirmed that some of the unencrypted customer information stored in the database was compromised, potentially including Second Life account names, real life names and contact information, along with encrypted account passwords and encrypted payment information.

, private data of 642K users was exposed. SecondLife has invalidated all Second Life account passwords and users have to reset their passwords. The exploit has been patched.
As we engage more and more in Social Networking, I believe it is important to understand what private data could get out on the streets before you sign up. Are we getting careless with our private data ?