I’m not sure why Facebook doesn’t have this as a feature. Using this script you can set up Facebook to send you alerts when your friends are no longer your friends. It works by checking your friends list at regular intervals and then comparing the lists. I’m really surprised no one had come up with anything like this before. If I cared about Facebook more I would use this. It would have been good to have about five years ago.
Someone let me know if they actually use this.