Pew Internet & American Life Project: Rating systems

Twenty-six percent of adult internet users in the U.S. have rated a product, service, or person using an online rating system. That amounts to more than 33 million people. These systems, also referred to as "reputation systems" are interactive word-of-mouth networks that assist people in making decisions about which users to trust, or to compare their opinions with the opinions expressed by others. Many Web sites utilize some form of this application, including eBay, Amazon, Moviefone and Amihot.