Up to now I've been very slack about joining social networking sites. I'm time poor and just never get around to it.
Still, I've heard that they can be fantastic for traffic generation, and good for SEO too. For instance, simply signing up to many of these sites will get you a backlink that counts for SEO. Here's a good list of them.
Google Profiles is not on the above list, but is worth joining for the same reasons.