I never stopped to think of it. I've joined several other message boards when I had a question pertaining ot whatever the board was. I posted, got my answer, never went back. So what if that's what people do? Not everyone is into sharing their lives with the whole internet world. Perhaps they just wanted to find out a specific thing about their pet... got their answer and moved on.

Also consider: not everyone is seen as being online. I myself am set to invisible. I show up as a "guest" when you check out whos online.