I've been so shy most of my life, even online! Even when I'm posting anonymously on sites I'd worry about upsetting people or looking like a fool. Like Artesia, so often someone else has already said (usually far more eloquently) what I would have said, so I don't bother. Just posting 'Yeah, I agree with this' sometimes feels silly (but it's really not).
I've started so many posts & replies, then decided it sounded terrible & cancelled them, that my post count would be at least 2x larger (if not 3x) if I had gone through with them.
I'm getting better about it, because I've realized that even little things like congratulating someone's achievements or good fortune helps them a great deal. Maybe it even helps those who are reaching for those same achievements (wait, did I start talking about video games?).
I started my own diary thread, and while I generally only post to it once a week, it's been very good for me to post how I'm feeling or what I've been doing. You might try doing that yourself!