101 Million Americans Subscribe to Amazon Prime Inc. (NASDAQ: AMZN) reached another milestone in the fourth quarter of 2018. The e-commerce giant’s Amazon Prime membership count reached an estimated 101 million, according to data released Thursday by Consumer Intelligence Research Partners (CIRP). About 62% of Amazon customers are now Prime members.

In Amazon’s annual letter to shareholders in April of last year, CEO Jeff Bezos said that the company had more than 100 million Prime members, but that was a global total. That was also the first time the company had ever released a Prime subscriber count. CIRP estimated at the time that Amazon had about 95 million U.S. subscribers.

Year-over-year growth in U.S. Prime membership came in at 10%, below the 12% annual growth posted in 2017 and far below the 35% growth registered in 2016 and around 50% growth in each of the two prior years. But CIRP co-founder Josh Lowitz noted:

Membership has slowed, but continued steadily in the holiday quarter. US membership grew 10% in the past year, slower than before, but still significant on a huge base and after years of rapid growth. Membership doubled in three years and almost quadrupled in five years, from 26 million members in December 2013.

Co-founder Mike Levin added:

As Prime membership growth flattens, the nature of the membership changes somewhat. One-third of members pay a monthly fee, and can basically leave and rejoin Prime at any time. They do this even though the annual cost of $119 costs less than the sum of twelve monthly payments on the monthly plan, or $156. These more transient members obviously don’t have the same commitment to Amazon shopping and the suite of Prime member services. Presumably, they don’t typically use the breadth of benefits to the same extent as annual members.

CIRP also reported that Prime customers spent an annual average of about $1,400 with Amazon last year compared to non-Prime members, who spent about $600 a year. A Prime member spent an annual average of about $1,300 in 2017, while non-Prime members spent about $800.