I just joined the facebook - main reason was to brag about my new baby to people I haven't seen in decades and make them look at pictures, but also I think it's good to maintain a good network, for job search reasons. Lots of my college friends are from my major and have similar jobs; I have found former co-workers, etc. The easiest way to find a job is from personal networking, and I want to make sure that the network is healthy before I need it.
HS friends - well, that might not be as fruitful of a network, but you still never know. One girl I knew from way back is an HR director at a company...good to keep in touch with. It is kind of cool to see what happened to everyone I used to know though...kind of like a HS reunion without the awkwardness.
One general observation - Man, I'm glad I moved out of my hometown. Reading old HS friends messages, seeing pictures, and reading the info section...makes me a little less nostalgic about moving back to that area.