LinkedIn is supposed to be a higher quality version of social site, b/c it's invitation-based and targeted at professionals who want to make job-related contacts.

I'm not sure how well it works. I only used it once, as I was trying to locate a friend and Google directed me to her LinkedIn page. In my field, we generally know where our colleagues are and what they're up to, and who to contact for whatever questions we might need help with. But I assume that is not the case in many fields, so it might be quite useful to form a network of contacts, especially if you're in the job market. What do you have to lose? Give it a try and see.