During Tuesday's Prime Day sale, Amazon will be scouting new members to enroll in its annual subscription service. But as the number of people paying for unlimited free two-day shipping grows, the pool of non-subscribers is getting more shallow.
New data released hours before Prime Day estimates Amazon has added 19 million Prime members in the U.S. since last year's inaugural event, when it lowered prices on an array of items to celebrate the service's 20th birthday. That equates to growth of 43 percent, for a total 63 million members, according to the figures from Consumer Intelligence Research Partners.
And with membership growing 9 percent in the three months ended June 30, the second quarter marked the first time a majority of Amazon's U.S. customers were Prime members, the research firm said.
"Amazon promoted the service aggressively starting with the first Prime Day in July 2015, and continuing with exclusive product offers and enhanced services throughout the year," said Josh Lowitz, partner and co-founder of CIRP.
That includes expanding its membership options to cater to shoppers who aren't ready to commit to a full year. In April, Amazon starting offering a monthly Prime subscription for $10.99, compared with an annual membership fee of $99. About 22 percent of the quarter's new enrollees chose the monthly plan, CIRP said.