I was inactive for quite some time after initially joining the forums. Mostly because I didn't have much time (avoiding showing parent's and sibligs, Now all but my dad and sister know) and didn't find all that much to talk about, so I lost interest for a while.
Then as I said, most of the family knows (and honestly I stopped caring). Plus I started to hang out with @Kolth more IRL and used this to get in touch with him. There are quite a few more reasons, but I don't feel like taking the time to write them down.
As for why others disappear, I honestly don't know, but would assume it's for similar reasons.