Facebook Friends Checker tells you when someone defriends you on Facebook

If you're obsessive about your Facebook friends and you always notice when your friend numbers go up or down, Facebook Friends Checker might be for you. It 's a Chrome extension which keeps track of your friends and periodically informs you when someone defriends you or deletes their account. You'll get an alert on top of your Facebook page, showing your lost friends and it'll stay there until you dismiss it.

Note that you won't see Friends Checker working immediately because it's not checking your friends list every second. Don't worry, though, I promise it works. There's also a Greasemonkey script, which will work on Firefox and Safari (with Greasekit), but I didn't have any luck getting it to work.