Mike Blumenthal just posed a question about this on G+ and I thought it was worthy of posting here. The TL;DR answer is:
1. On G+, Yelp and most other review sites, the reviewer typically owns the content of the review (this is to shield the review sites from liability) while the review sites own the right to do whatever they want with the content.
2. Third parties typically cannot use the reviews on their own services without consent from either the service or the content owner (aka the reviewer). Yelp has an exception for non-commercial use, whatever that is.
3. So if you have a business you should probably include in your TOS something like “You agree to give us a perpetual, royalty-free license to use any content, such as a review, that you create related to our business, however and wherever we want.” or something legalistic like that.