Most of us are at the point now where if a new social network comes into our lives, it means getting rid of an old one — Flickr -> Instagram, for example. That’s a pretty big problem for new social networks going forward. They can’t just be good, they have to be really, really good to make up for a switching cost. Or they have to be totally different — but even something totally different means time spent there instead of elsewhere. Something will probably be cut.
Your service now can’t just be a time-waster, it has to be good enough to make the cut.
Plus with any new social network, it only takes off when your friends join. Case in point—Path. Gorgeous UI, fun to use, but I only really got into it when more friends got on it.