There's a pretty serious article about Yelp, which has become the dominant player in restaurant and
service reviews in the SF Bay Area, in a local alternative weekly The East Bay Express called Yelp and the Business of Extortion 2.0.
Now recently Yelp has seen a couple of its reviewers sued for reviews about health care providers (both chiropractors), and the issue about what reviewers can say online is probably still to be worked out.
But this article is about something much worse. It accuses Yelp of changing reviews, eliminating them, and generally breaching the church/state line between community and sponsorship.
And it goes both ways. Businesses that advertise get bad reviews “disappeared” and those who don’t find their good reviews are vanishing.