how many small, offline businesses participate on Yelp? participating on some location based service seems like it wouldn't be any more difficult.
takes time to build a comprehensive, online local search presence... particular one with good social coverage and activity.
It's possible that the best way to engage with small offline businesses is to hire a small (okay,maybe medium or large) army of salespeople, and pitch to each business individually. If that it the case, you are absolutely correct. It will take a long time and an incredible amount of money.
However, I think it's possible that someone may come up with a better, cheaper way to do it. I don't know what that better cheaper way is, but I'm not convinced that an army of salespeople is the only solution to that problem.