I'm thinking that they do it in order to get involved in something they identify with, but there's just so many groups and stuff on here that it's hard to navigate through them all and stay on top of it (unless you are just on the computer all the time). I dropped some that I signed up on in the beginning, but hell, I'm finding that I don't even get around to the ones I've CREATED enough, much less any of the others.
I think if we can get something compelling in here, they will come. Maybe.