Amazon.com's (AMZN) annual Prime Day has become a big hit, but not just for the sales it generates on the day itself.

Sure, the e-commerce giant will sell billions of dollars worth of goods, including plenty of sale-priced Echos, Kindles and other electronics, as well as deals at Whole Foods for the first time, on the now-annual celebration of Amazon's birthday. But the real pay-off comes off in the number of people who sign up for its Prime membership program in order to take advantage of all the bargains. 

These members, who receive free two-day shipping on many goods, plus streaming video content, music and other perks, are Amazon's most loyal customers, spending almost five times as much as non-Prime customers, according to Morgan Stanley research. They also generate billions in recurring subscription revenue each year for Amazon, although the company spends a lot of that money to provide free shipping, of course.

Last year, in addition to posting its best-ever sales day on Prime Day, Amazon also said it added more Prime subscribers than on any other day in the company's history. Here are some of the key numbers to keep in mind as this year's Prime Day approaches on July 16. 

Amazon is a holding in Jim Cramer's Action Alerts PLUS Charitable Trust Portfolio. Want to be alerted before Cramer buys or sells AMZN? Learn more now.

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