BuzzFeed has spotted "Who Deleted Me," an app that shows users who has unfriended them or deactivated their Facebook accounts. It has been added as a browser extension for Google Chrome, Firefox, Opera, or the app can be downloaded to iOS and Android devices.
It will not, however, tell you why you got de-friended. Presumably you have better things to do than bring it up with these people, whom you probably do not talk to anymore anyway.
There is a similar app, Who Unfollowed Me, which tracks who unfollows Twitter users.