I like LinkedIn. I joined less than a year ago but I've already logged considerable time on it, sending invites and exploring its features. I've used it primarily as a way of keeping track of my business associates, and since I work at home, keeping track of my associates online is a natural. Here's a good article on how to make the most of your social networking site membership. I must confess that I don't always follow best practice for sending an invitition, but I'm going to get better at it now that I've read this article.
One of the really powerful features is the support LinkedIn gives for job searches. Specifically, how to find and connect with hiring managers in the companies you've targetted for a position. By now, everyone knows that you don't send your applications to HR, you try to make contact with hiring managers. It's not that easy, though. However, LinkedIn gives you a way of finding your connections to those individuals. It supports good practice for finding a job, and as a designer, I appreciate the thought that went into doing something this well.