No, you're not the only one. I'm very much the same as you and understand completely what you are saying. I tend to be rather quiet (not to be confused with shy) in "real" life. Get me in a group of strangers and I'll be the one listening to everyone and only occasionally speaking up, if at all. I seem to be the same here, posting only once in a while, though I've been around lurking for over three years. I don't really know why. It's just more comfortable for me to read what others write and then log off.
Judging by the numbers of times a thread is read compared to the numbers of postings in that thread, I'd say there are a lot of quiet lurkers out there.