Sean Bonner nails it on the head about Qwitter, a Twitter-oriented (but not affiliated) service that informs you whenever someone stops following you:
My biggest problem with Qwitter, and the one that covers all the bases in one shot is that is creates drama and negativity from something that should be innocuous. Can you imagine if there was a service out there that notified you when someone you know chose to move an e-mail you sent them from their inbox to their archive folder, or worse to delete it?
To me, there is and has only ever been one useful purpose for Qwitter that doesn’t create drama nor negativity: knowing who you can no longer send Direct Messages to. That problem could be easily alleviated though if Twitter itself would just inform you when you try to send a DM to someone that’s not following you and your DM gets lost in a black hole.