I don't use Facebook and I never will. It is a great source of humor though.
I have a Twitter account that I've had for 2 years and I have made exactly 0 tweets. I use it as a news aggregation for people and sites that I want to stay up to date on.
LinkedIn is the only network that I have a real account on but it is for professional, business use only. I figure one day if I ever switch jobs it will be useful for me to have kept track of my colleagues and contacts.
For a multitude of reasons, I have no interest in keeping people up-to-date on my life. The people who matter most to me will know the pertinent details any way either by living with me, being at work with me, or telephone/e-mail.