Engineers and social-networking sites

It seems engineers are increasingly using social-networking sites. I've even heard that PLM companies are considering embedding social elements such as microblogs and Wikis into their systems. Do you use any of the sites? If so, what are the advantages over more conventional means of communication? (I use Linkedin, Second Life, and Twitter, as well as a few IMs). Twitter, for example, is a nifty way to communicate with colleagues anywhere in the world, off-the-cuff.