Please be wary of Tagged.com. If you let it supposedly check your contacts for people who're already on the social network, it will email an invitation to everyone in your address book. I remember when Hi5 did this to me a year or so ago. It spammed everyone I knew, including my former headmaster. The emails began, "Hey!" and boasted "I now have over 13 friends in my network!"
I doubt he was much impressed. Meanwhile, the blessed people at Tagged explain, "Advertisers love Tagged because they get clear, uncomplicated access to our audience."