So here's a scary fact: 41% of Facebook users are ready and willing to give away personal information to a complete stranger. No, not preferences about what they're looking for in a mate, what their secret vices are or how they have an out of control foot fetish. I mean the type of info that could leave them vulnerable to identity theft.
4 in 10 Facebookers added "Freddi Staur" to their contact list, a fictional character whose avatar was a plastic frog. Sophos, an IT security company conducted the experiment using the frog ("an anagram of 'ID Fraudster'") to befriend people while giving out very little detail about himself. 200 friend requests later, and the company was astounded at the amount of personal info they were able to collect.
Even if they didn't give out information through direct communication with the frog, there was enough information accessible once the frog had been invited to view the user's profile that they could have used it as to gain any further data. This includes identity theft as well as letting the cybercriminals infiltrate entire computer networks.
Check out the Sophos site for their tips on Facebook Best Practice.