I doubt they would pay people to review, unless it's a well kept secret. What they do have are Community Managers. They are located in each major city prompting Yelp, providing support for the yelpers in that city and so on. They probably spend a lot on their sales team as well.
I highly doubt Yelp pays people to review...anymore. They did when they first launched (they've admitted to this), but with their brand recognition now, paying people to review would be a giant waste of money. That being said, they haven't reached profitability yet, so who knows, maybe they are making this bad decision.
