I think I joined because I saw a really interesting discussion on the little sidebar while watching ZP. I'd been watching them for a while, but was never aware of the message boards.
I joined, I posted, then during the summer I didn't really go online. And... now I'm back! yay!
I don't really lurk, if I have an opinion I like to say it. (Completely opposite from me offline). I usually hover around in topics with few replies, because I know the feeling of "no one likes my topic =(" and I want to make people happy. That, and I have a hard time keeping up with 10+ pages of discussion.