Stay away from the apps on facebook that claim to notify you when someone unfriends you. They're against Facebook's privicy policy, so the ones that do work (if any) are quickly removed. The rest are purely there to steal your personal info and your friends list, it's part of a black-hat marketing technique.
The best method of finding out who unfriended you on facebook, by far, is unfriend finder. It's a browser add-on that will notify you when someone unfriends you or ignores your friend request, just like the notifications you get when someone sends you a message or adds you as a friend.