To start using this tool, you need to do a scan so the tool can save your friends list, which it will later compare with a future version to detect any changes. That done, and once you've added the app as a Chrome extension, you'll see the Who Deleted Me icon accompanied by a red X, indicating that nothing has changed. If someone does unfriend you, a little number will appear there to show you how many people have done so.
The good thing about Who Deleted Me is that it notifies you when you lose a friend on Facebook and also specifies who that person is. To show you exactly how many people have unfriended you, this application checks the first scan that you did and looks for any changes. However, if someone deletes you but then adds you back before you check the application, you won't be notified.
The webapp will also tell you how much time has passed since the friendship ended and who ended it (since it may be you who did the unfriending and not the other way around). Thanks to this interesting Chrome extension, you'll be able to keep track of all the people who've decided to unfriend you on this ubiquitous social network.