Well, I've been a member on this site for nearing on three years now, so I like to think that may give me some credibility, despite my, err, less than impressive post count. There are a couple of reasons it is so low, however.
First is that I generally don't do well with forums or internet communities in the first place. Never have, really. There are few places where the collective idiocy of humankind is as glaringly apparent as it is on the internet, and staying off of forum platforms at all is one thing that maintains my very fragile little flame of hope I have left for humanity. The fact that I am a woman and therefore inevitably more of an easy target for vitrol-spewing morons does not help. This way I keep myself from wanting to snap someone's neck every few minutes. Well, usually.
Second is that I just generally have never been too fond of communicating within the boundaries of text (despite my love for literature). I am a fairly social person. I like to talk with people and have a very easy time to make new friends, yet I feel rather uncomfortable speaking via the internet. It's much less personal, and you miss so many important aspects of communication this way. Tone of voice, body language, personality... On the internet, I don't know who I'm talking to, if anyone will ever even listen to what I say, and it's that much harder to tell how serious the other person is. No, I'd take speaking to people's faces in real life over a message board any day. It's just kinda hard to do, even though there are conventions arranged for that kind of stuff, because I kinda live in Sweden. I also don't have an income right now.
The fact that message boards tend to be completely riddled with word crimes all over does not increase my willingness to use them as a platform, either.
Lastly, and perhaps most importantly, I simply don't feel like I have anything to say, most of the time. Either I'm not invested in the topic, or there's too much whining and yelling going on, or my point has already been said fifteen times by other posters already. I may be a chatterbox when I speak with someone face-to-face, but even then, and even more so on the internet, I strictly go by the code of "If you have nothing to say, say nothing".
The few posts I have put up so far are all times when I felt like I had something to point out that people, from my point of view, seemed to be missing, or when I felt I otherwise had something to contribute with, if only to supply my opinion in a topic I cared about.
I guess what I'm trying to say is that there may be a good reason behind a person's low post count. One should go by the content of the person's post, because that's all that really matters, in the end.