Amazon.com's (AMZN) loyalty program, Amazon Prime, has penetrated nearly 40% of the U.S. market, with some 41 million members, according to a Cowen survey of 2,500 U.S. shoppers that the investment bank released in a research report Wednesday.

In December, membership had risen 32% from the previous December, said Cowen, bolstered by Amazon's shopping "holiday" over the summer dubbed Amazon Prime Day. The one-day promotion drove thousands to buy the $99-a-year loyalty program, which includes perks such as free two-day shipping, access to streaming video content and free e-books. Prime Day added 1% to Amazon's overall sales, the company said on its Q3 earnings conference call.

Prime shoppers buy more than non-Prime shoppers.

Wells Fargo analyst Matt Nemer, asked to identify one reason for Amazon's ongoing growth, pointed to Prime.

"That's their weapon right now (fulfillment)," Nemer told IBD in an interview about Amazon's competitive position in the market. "The extremely efficient distribution program feeds into Prime. There's a lot of loyalty."

Amazon didn't invent the loyalty program, Nemer says, Costco did. But, he says, Amazon is out in front of other e-commerce platforms, in part thanks to Prime.

"I'm surprised we haven't seen more innovation in loyalty, but I don't think it's too late," he said.

The Cowen survey, looking at e-commerce overall, said online sales of electronics and general merchandise (EGM) rose 23% in December from December 2014, down from 26% growth in both November and October.

Cowen analyst John Blackledge wrote, "net-net Q4 purchaser growth trends remain strong and bodes well for Q4 2015 results, in our view."

Cowen said Amazon's online apparel purchases rose 25% in Q4 vs. a year earlier, but it fell 7% and 6% at Wal-Mart (WMT) and Target (TGT), respectively.

Amazon stock was down 5% in afternoon trading in the stock market today, near 586, but shares more than doubled in 2015, touching a record high near 700 on Dec. 29.

Analysts are mixed on the e-tail giant. Several are bullish, mostly because of Amazon's cloud computing business, but others see less near-term upside for Amazon stock. Some also think that Amazon is going to build a delivery business and sell excess capacity — much like the firm has done with Amazon Web Services, its cloud division.