Where do I get the first sign-ups for my social network?
anigbrowl: Advertise? Building value for one or two niche markets (on the same platform) first seems like a good strategy. If you have some unique benefit then you can go after known enthusiast markets - games, creatives,  etc. or even niches within niches. Obviously, your goal is to offer something sufficiently cool and useful that users will want to invite their friends.