Some people get tired of the site and leave.
Some people get pissed and leave.
Some people leave and come back.
Some people leave and secretly come back.
Some people get kicked out (which you have to work pretty hard at, it doesn’t happen lightly).
Some people complain about what a terrible site it is, think that they shouldn’t have to follow the same rules that everyone else follow, get kicked out, and then keep coming back to complain about how terrible it is here (?).
Back when we were first given the option to “follow” I put a few users in “my Fluther”, until I decided that I didn’t like the popularity-contest-feel and stopped participating. I recently noticed that at least half of the members in my list are gone or are no longer active. It happens. It’s their prerogative.