Last year, a New York steakhouse took legal steps to force Yelp to identify who wrote a certain post on Yelp. The writer claimed to be a waiter habitually spitting into the food served at the steak house. The writer used the name of a real person who said he had nothing to do with it and apparently filed a police report about the matter. The steak house took Yelp to court in an attempt to track down the real poster. (Read more here, here and here.)
Earlier, another company had taken a similar matter to the courts in Virginia, claiming defamation. These cases force the US courts to carry out a delicate legal balancing act. The Virginia Appeals Court initially ruled that Yelp had to reveal the identities of seven posters, but Yelp appealed against that decision (read more here, here and here). The Virginia Supreme Court heard the matter in October 2014.
I haven’t been able to find recent information on the internet about these cases so it’s not clear to me how either of them ended. As there is a great deal of debate about the validity of Yelp reviews and the company’s ability to manipulate reviews, the point may be moot.