i like to think when people leave the site, it's for a good reason.. they've got better, or they are busy with their lives.

maybe the friend thing is part of it.. for someone shy, for example, friends can be hard to make (even on sites like this)

maybe some people delete because they might be read by guests, and others who they don't want to see it- i've seen a couple of threads where the original poster has said something about privacy

and like was said, maybe some will come back under diffrent names