If one of your Friends installs/uses an application, that application is given the same access rights to all of their Friends data (one of them being you!) as they have. This gives the application the opportunity to copy and store very personal information such as date of birth, address, telephone number(s), employment and/or education history (assuming you’ve added it and not limited the access to it). So for every silly, pointless quiz/game that one of your Friends installs/uses it opens all of the data on your profile up for it to misuse.

There is a very simple fix to this though. Delete your profile, all your friends or wait until next year sometime when Facebook implements more stringent controls on data access for applications…