I agree with Martin. The best way for these things to get going is to figure out a way not only to meet new people but to bring existing friends into it. It has to be something so spectacular that it beats IM (or compliments it directly). I remember when I signed up with Friendster - I invited about 20 "real-life" friends and although over time, about 70% actually signed up, I think that only 5% of those 70% ended up inviting other people and/or using it themselves.

So yeah, social networks may be cool but for the moment that's all they are. And because cool does not necessarily translate into marketable, I don't see what the big deal is.