It's not that easy... For extortion, the punishment you're threatening someone with also needs to be illegal. For example: "Pay me 100$ or I'm going to tell all my friends how bad your food is" isn't extortion.

I'm not sure about the actual situation with Yelp, but as long as they're not making up negative reviews (which could be slander), it might be ok, legally. Given their size in that market, anti-trust concerns might come into play, though.