Using the 37 signals line you quoted. The key word you missed was solved. Like you worked out this doesn't solve the problem because there aren't enough users using it. That's another problem to solve.

The best advice from the thread already target a specific city. Lock down that city then move on (I wouldn't choose SF as already so many geo mobile startups targeting it, also SF users maybe don't represent the broader market).

Also leverage existing API's. Or hell become an API combining all other sources and let other people build the apps (seems to me the real problem, billion people connected to different networks). If I look on meetup maybe I'm missing out on great events from other sites, but I don't have time to search them all.