Does Facebook tell your "friends" that you have deleted your account when you leave? If not it seems like the sort of thing many people would want to announce somewhere.
It was kinda entertaining how so many people assumed that something major had to have happened for me to get rid of Facebook. Really it just started as a bet and then I never got back on.
still it's not about announcing lack of a specific communication channel (most of 'friends' don't care anyway), as I glanced at a few of them it's more about making a scene (facebook would be a good place for that. zing!).
I know, it doesn't have much utility, but it would be incredible.