Using this app is no big deal. All you need to do is get your self registered to their service and sync your Facebook account with it. And it keeps the track of all you activities on Facebook. So whenever there is some change in your friend list on Facebook, you get a notification via email.
Who deleted me on Facebook
does not offer you with any of the previous information. It infact starts functioning from the moment you sync your Facebook account with it. So start using it and find out who unfriended you on Facebook.Happy Facebooking!!