There's tons of websites, Pages, and Groups on Facebook claiming they will show you who unfriends you, but they're just trying to scam you. Unfriend Finder is, however, the real deal (until Facebook shuts it down, of course). It is available in 65 languages and works with Microsoft Internet Explorer, Mozilla Firefox, Google Chrome, Apple Safari, and Opera. You can grab the plugin from UserScripts. To see how to install it for Chrome, how to install it for Firefox, and what it offers, check out the following Photo Galleries:
If someone accepts a friend request from you, Facebook sends you a notification, both on the social network and via e-mail (the latter you can disable). If one of your friends deletes (or unfriends) you, Facebook does not notify you. The company says so explicitly in a Facebook Help Center entry:
If I unfriend someone, will they be notified? No, but all friend links are reciprocal, so you will be removed as a friend from their friend list as well.
Facebook does this so users don't feel guilty when they unfriend someone. Unfortunately, some users just can't help themselves and are unbearably curious who is unfriending them, and when.
That's where hacker Edouard Gatouillat comes in. He has developed a browser plugin called Unfriend Finder that shows you who has unfriended you on Facebook. Unfriend Finder won't show you who has unfriended you in the past, but it will work for anyone who unfriends you going forward. It checks who has gone missing by constantly monitoring your Facebook friends list.
Here's the plugin's official description:
Unfriend Finder is a realtime script that allows you to know wich one of your friends removed you on facebook. A locale database keeps record of your friendlist, and is compared at each refresh. Unfriends are people who were previously your friend but have either deactivated their account or have removed you from their friendlist. Unfriend Finder will tell you which friends have removed you, who has deactivated their accounts, people you have requested friendship with and who has declined your friend request. Unfriend Finder makes identifying unfriends easy by displaying a red counter in the menubar with the amount of unfriends you have. It also uses Facebook notification to alert you on these events.
Please note: This script can not tell you which profile(s) had you on their friendlist before it was installed. Unfriend Finder is not retroactive.
If this plugin stops working for you, please let me know.