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By Matthew DeLuca

About half of all Amazon’s American customers now subscribe to the online giant’s annual Prime service, with 14 million new members joining in 2015, according to new estimates from Consumer Intelligence Research Partners.

The new estimate pegs the total number of U.S. Amazon customers who subscribe to Prime at 54 million. And those members tend to shell out more money on the site, spending an average of about $1,100 every year – nearly double the purchases of non-Prime members.

CIRP based its analysis on surveys from 500 U.S. purchasers who bought something on during the holiday season, between October and December of last year.

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Prime subscribers pay a standard rate of $99 a year for a package that includes expedited shipping, music and video streaming, and special deals on some products, among other services.

While netting such a healthy number of subscribers underscores Amazon’s online retail hegemony, the company will have to move its focus from signing on new members to the challenge of keeping the ones it has, CIRP said.

“While Amazon Prime continues to grow nicely, growth slowed somewhat compared to earlier periods,” Mike Levin, CIRP partner and co-founder, said in a statement. “We expect this, as the program matures in the U.S. So, member retention becomes even more important.”

Amazon is generally tight-lipped about how many customers use its Prime service, with some rare exceptions. The company said late last year that it had added 3 million Prime members around the world during the third week of December alone. Prime membership grew 35 percent in December compared to a year earlier, according to CIRP’s analysis.

An Amazon spokeperson would only say that the company has "tens of millions of Prime members" on Wednesday.

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