Amazon (NASDAQ:AMZN) has been kind enough to offer a glimpse into Prime's growth for the last few years. It said membership grew 53% in 2014, and another 51% in 2015. Even the more saturated United States grew membership 47% in 2015. Amazon neglected to give an update in 2016, but it did change its revenue reporting, giving us a closer glimpse of its numbers. I used those numbers to estimate that Amazon had around 70 million global Prime members on average throughout 2016.

Well, that number has only gotten bigger. Consumer Intelligence Research Partners (CIRP) estimates Amazon has racked up 80 million members in the U.S. alone as of the end of the first quarter. That's double the 40 million members CIRP estimated at the end of 2014. What's more, growth accelerated quarter over quarter compared to the same period last year. While growth will eventually slow, Amazon CEO Jeff Bezos seems to be doing a good job of delivering on his goal with Prime: "To make sure that if you are not a Prime member, you are being irresponsible."

Warehouse worker inspecting boxes coming down a conveyor belt.

Image source: Amazon.

Accelerating growth

Despite its huge member base -- 60% of all U.S. Amazon shoppers -- Prime is actually experiencing faster growth than it did last year. "Membership grew 8% in the most recent quarter, compared to 7% growth on a smaller base in the year-ago quarter ending in March 2016," said Josh Lowitz, Partner and Co-Founder of CIRP.

One of the biggest contributors to Prime's membership growth is the introduction of the ability to pay monthly. Amazon introduced the monthly payment option last April. CIRP estimates that 26% of Prime members pay $10.99 monthly instead of $99 annually.

While the monthly option carries the risk of fewer members keeping their subscriptions (as they have more opportunities to cancel), CIRP found the monthly payment plan actually improves retention. "We think that the monthly membership option appeals to the later Prime adopters, with a smaller, potentially temporary commitment, that ultimately yields a long term commitment," Lowitz said.

Another factor may be Amazon's increased investment in Prime Video. Amazon doubled its content spend in the second half of last year and released three times as many originals. The investment paid off as video consumption doubled in 2016 compared to 2015. Amazon also won several awards for its original films and television series during awards season this year, drawing even more attention to Prime last quarter.

Amazon's financial results back up CIRP's estimates

There are a couple of major indicators of Amazon Prime's growth in Amazon's quarterly report.

The first is unearned revenue. Amazon charges $99 for annual Prime memberships, but it doesn't count all $99 toward its revenue right away. Instead, that revenue is spread out evenly throughout the 12 months. The cash it collects is balanced by a line item called unearned revenue.

Last quarter, Amazon's unearned revenue climbed to $4.1 billion. That's up about 45% from the $2.8 billion balance it carried last year. Granted, not all unearned revenue is related to Prime. Amazon Web Services -- the company's cloud computing division -- also contributes to the number.

The other indicator is Amazon's retail subscription services revenue. That item increased 52% year over year, excluding impacts from foreign exchange rates. That's a marked acceleration from the fourth quarter, when the number grew 33%. When asked about whether Prime membership growth was a driving force behind the acceleration on the earnings call, CFO Brian Olsavsky specifically declined to comment. He only said that strength in the fourth quarter translated into strength in the first quarter.

Overall, Amazon Prime is still generating a massive number of new sign-ups. With 60% of Amazon's U.S. customers subscribing, it still has some growth left, but it will eventually hit a wall. That's why Amazon is aggressively investing in Europe, too, where its Prime membership and overall share of the e-commerce market is relatively small. As Amazon grows Prime, investors can expect it to capture still more of the growing U.S. and global market for online commerce.

Adam Levy owns shares of Amazon. The Motley Fool owns shares of and recommends Amazon. The Motley Fool has a disclosure policy.