I use the following sites, and although I haven't been swarmed with clients from them, I think I have benefited from each in form or another.
MySpace (have received several inquiries and 1 new Meetup member)
Linked In (have received several inquiries)
Active Rain (received one RFP)
Merchant Circle
MeetUp.com - Entrepreneurs meetup (1 client)
Cre8buzz.com - Just joined (invite only, if anyone wants to join, pm your email and I'll send invite)
Facebook.com - Just joined
I am also a member of several industry-specific forums, which I have received inquiries from and a client. I check each site daily and post on the various forums, but I rarely blog because I hate writing. I think I have received good responses from each seeing as how I've only been up and running since July. Whether I receive responses or not they all provide quality backlinks which have helped with
SEO.