I find they're good if you understand their limitations and only try to use them for what they do best.
For example, I'm on Facebook and setup a "product page" there. I've even gotten a few "fans" who aren't my friends
It allows me to setup an event (which I use for open houses and exhibitions) so I can invite everyone to it and use it for advertising. I don't think it could ever completely replace all other methods of advertising, but I find that it helps.
Gallery/photosharing sites like MyArtSpace or Flickr are good for putting your art out there, but you still need to direct people to your page. I know many people who are frustrated that they haven't been discovered yet. Just remember that so many people are out there that you probably have a better chance at winning the lottery than being accidentally discovered!
I find that niche social networking sites like Twitter or MyArtSpace are great for finding your peers, but will probably have little effect for getting noticed. Chances are with micorblogging sites like Twitter, people won't follow your updates unless they already know who you are. Good for retention, not as much for recruitment.