To permanently delete you the account, do the following :
1. Go to http://www.facebook.com/help/contact.php?show_form=delete_account ,
2. Click on SUBMIT only if you are sure you want to delete the account.
3. Once you click on Submit, you will see the below pop up: Permanently Delete Account
4. Just click Okay. Voila! bye bye facebook.
Note: If you decided to change your mind to re-activate your a/c you can do so by just signing in within 14days.