This is hearsay; I wasn’t in the room at the Xmas party where it was said. However a close friend’s wife is a hair stylist (also a recipient of a Google Favorite Places decal) and he reported to me that the salon’s owners estimated that roughly 80% of internet-based new business was driven by Yelp.
Admittedly this is the Bay Area where Yelp is strongest but that’s a massive number and illustrates the site’s power.
Meanwhile Google has added more reviews/sentiment analysis to Place Pages:
This is helpful but not as useful for example as Yelp’s review summary capsule, which does a better job of providing at-a-glance information:
Nonetheless Google’s move to make Place Pages more useful does step up pressure on Yelp and others.