I use
MySpace but it is strictly professional. I have a separate page for friends/family that I have set to private. I have made some good contacts through there, but their forums are pretty worthless, just full of spam. To avoid spam on my profile I check each and every profile before accepting them as a friend. When searching for new contacts I search for people in my target market and then sent them a friend request.
I am on a lot of sites, Linked In, Start-up Nation, Xinga, Facebook, Merchant Circle, etc. I have had made the most good contacts through MySpace.