If think the point being made is Amazon didn't suffer the same fate as Yelp wrt to book reviews because Amazon had a better product than whatever Google produced.
I know a few people in the restaurant industry (owners/managers). They all say the same thing: at best engaging yelp ( and advertising on yelp ) is a non-negative. It does not bring enough new customers to justify its expense.
For about the same amount that yelp wanted for sponsorship, they got over 30 real, quality reviews that complimented the recent improvements that had been made.
Yelp sucks so not a top link. Seems to be working in a fair manner.