How does this work? I just tried clicking through and it returned a list of friends who added me.

If there was no list of people who deleted me, does that mean it hasn't happened? Was it working properly?

I'm going to assume that it saves a record of your friendships when you first login. Then every time you come back to the site it looks at your current friend list and tells you the names of the people that dropped off.

If it does work this way, this is a breach of the Facebook developer terms of service. Data may be cached for specific amounts of time, but long term storage is not allowed. When this app gets big enough to notice, Facebook will shut it down

This is not true: https://developers.facebook.com/policy/#data

> You may cache data you receive through use of the Facebook API in order to improve your application’s user experience, but you should try to keep the data up to date. This permission does not give you any rights to such data.

Although it's not specifically in their terms, FB has a history[1] of trying to prevent unfriending apps (it's a cat and mouse game).

[1] http://socialfixer.com/blog/2013/10/27/facebook-doesnt-want-...

Yes, I'm aware of this. But the applications they went after adjusted Facebook (injected into the DOM), instead of leveraged their API to provide functionality through a Facebook canvas app, or externally.

Correct. It's a very simple idea.

I think you have to have it enabled and when someone disables or unfriends you on Facebook, you will be notified.

^ ditto