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Amazon.com Inc.'s (AMZN - Get Report)  Prime membership has grown to 101 million, a milestone number reached even as growth in the pricier discount club slows, according to Consumer Intelligence Research Partners.

Approximately 62% of Amazon customers now hold Prime memberships, spending $1,400 a year compared to $600 for non-Prime Amazon customers, CIRP reported.

Still, while Amazon expanded Prime membership in the U.S. by 10% over the past year, the pace of growth has slowed.

While Amazon added a little under 20 million new Prime members between December 2016 to December 2017, pushing the total past 90 million, growth in the past year has been roughly half that, CIRP numbers showed.

Even so, the growth is notable, said Josh Lowitz, partner and co-founder of CIRP, in press statement, noting it was "still significant on huge base and after years of rapid growth."

"Amazon grew to over 100 million US Prime members in the quarter," Lowitz said. "Membership growth has slowed, but continued steadily in the holiday quarter."

Overall, Prime membership has doubled in the past three years and quadrupled over the past five years, from 23 million in 2013, according to CIRP.

Amazon's stock price slipped slightly on Thursday to $1,681.88.