At my startup incubator they are always reminding me to talk to customers.

Why not try to track down 25 individuals and get them to sign up and offer something?

Once you apply what you learn from that and have a decent product then you should think more of the advertising and marketing, everything from posters to ads on local cable tv. "Organic growth" is not enough because you have to get a large amount of participation to make something that is (i) self-sustaining and (ii) replicable in other areas