Facebook doesn’t make information about who’s unfriended who public, possibly for fear of bloodshed. But there’s a browser extension called Unfriend Finder that lets you do just this for yourself.
Compatible with Chrome, Safari, Firefox, and Opera, this script by Edouard Gatouillat is simple to install: Download it, make sure the extension is active, and then, the next time you log into Facebook, you’ll get a series of instructions on how to use it, as well as a setup box. As you can see in the picture below, an “unfriends” menu pops up on the left-hand side of your page, below the “friends” menu. You may want to restart your browser after installing it as well, as I was initially grayscreened by Facebook when I logged in immediately after installing.
One major point: This app is not retroactive, meaning that it only tracks unfriends in the future, not in the past. So you’ll have to sit tight with it for a while before you can see any results, and the people who have unfriended you in the past will remain an intriguing, most likely unsolvable mystery.