NEW YORK (CNN/Money) - Customer satisfaction has taken a dramatic downturn after two years of steady increases, and the popularity of online e-commerce giants Amazon.com and eBay also took a fall, the American Customer Satisfaction Index (ACSI) said Tuesday.

For the fourth quarter of 2004, the ACSI index fell to 73.6 percent from 74.3 percent, the largest decline since 1997. The drop is due mostly to waning satisfaction with retail customer service, high gas prices and a glut of user-traffic on e-commerce Web sites.

The ACSI is conducted by the University of Michigan's Ross School of Business. It is also a closely watched barometer of consumers' brand perceptions, including those of online retailers.

The online portion of the ACSI survey dropped 3 percent to 78.6 percent, while customer satisfaction with Amazon fell 5 percent from 84. The world's largest online retailer also lost its crown as the leader in the e-commerce score index.

For online auction giant eBay, its popularity also fell 4.7 to 80.

But Barnes & Noble's online unit gained 1 percent to 87 percent to take the lead, according to the survey.

"Amazon (is) selling everything from garden appliances and apparel to electronics and used books," said Larry Freed, CEO of ForeSee Results. "But bigger isn't always better from a customer's viewpoint."

Barnes and Noble (Research) has stayed true to its business model and product offering, which makes it easier to service demand well and sustain higher levels of customer satisfaction," he added.

eBay's success has also attracted competition from merchandise liquidators such as Overstock.com (Research), Yahoo! (Research) Auctions and even Amazon.com, Freed said.

The e-commerce portion of the ACSI survey is conducted in partnership with ForeSee Results, an online customer satisfaction management concern.