I tend to lurk more than I post, usually because I feel that there are people who know more than me and/or can say something better than I probably could. Or what I was going to say has already been said, so no sense in repeating it. Plus, most of the time I get to spend online is at work, so it's easier to read a whole bunch of things when I have time rather than post a lot. Even in real life though I tend to keep my thoughts and opinions to myself unless I absolutely feel the need to say something. And I don't like to be the person that everybody disagrees with, so if I'm one of only a few (or the only one) with an opposing opinion of something, I'm probably not going to post it depending on the topic and situation.
But all in all, I'm just not a real big talker in real life or online unless I know you really well. I like to observe and chime in when I feel I can add something to a conversation.