If you're the insecure type who values your worth based on the number of "friends" you have in Facebook, then this simple script add-on called Greasemonkey can help you keep track of those who've unfriended you.

The script works with FireFox, Google Chrome, Opera, Safari and Internet Explorer. that notifies you when someone on Facebook unfriends you, alerts you when someone you’re friends with deactivates their profile, and helps keep track of your friend requests. It will show your pending requests, and give you info on who has ignored, or cancelled your request.