Some genius once said:
What makes FaceBook GREAT, makes it even WORST.
Guess what? That genius is nobody but exactly me. Seriously, have you noted that many users / developers create applications for Facebook? Daily hundreds of user created applications are submitted at FaceBook and you are using them without knowing them that whether those user created FaceBook applications are harmful or safe for your FaceBook account and even for your computer and sensitive data!
Consider this! If an application can ask permissions from you to post something at your wall and do stuff then in background it can steal your password. Infact I have a list of such user created applications which can steal your facebook password right away.