One in six Americans was a member of the Amazon Prime service at the end of the most recent holiday season, a 35 percent boost from a year before, according to a survey from a retail research firm released Monday. The report, by Chicago-based Consumer Intelligence Research Partners, says that nearly half of Amazon.com 's U.S. customers, or 54 million people, have signed up for Prime, a program that offers free two-day shipping for many purchases, as well as services such as streaming video and music.