Since its incorporation just over five years ago (Vic: can it only be that long?), Facebook has undergone a remarkable transformation. When it started, it was a private space for communication with a group of your choice. Soon, it transformed into a platform where much fo your information is public by default. Today, it has become a platform where you have no choice but to make certain information public, and this public information may be shared by Facebook with its partner websites and used to target ads.
If any of this alarms you, then go here and read how to opt out of the bits you still can.