Amazon for the first time has revealed how popular Amazon Prime is, saying the paid subscription service has attracted more than 100 million members worldwide.

The total subscriber count was revealed in a letter Amazon CEO Jeff Bezos sent to company shareholders on Wednesday.

"In 2017 Amazon shipped more than five billion items with Prime worldwide, and more new members joined Prime than in any previous year —both worldwide and in the U.S." he said in the letter.

Bezos didn't offer details to the membership growth, but his company has been expanding Amazon Prime to new countries including Mexico, Singapore and the Netherlands.

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Members who subscribe to Amazon Prime receive the main perk of getting free two-day shipping on eligible purchases. In exchange, they pay a flat fee of $99 a year of $12.99 a month.

Amazon started the program 13 years ago when an engineer named Charlie Ward pitched the idea through a company employee-suggestion box. Since then, Amazon has introduced other perks, like free two-hour delivery on certain items and access to Prime Video, the company's own streaming service.

Amazon has remained mum on the exact subscriber accounts for years. However, in recent months analyst firms had estimated that Amazon Prime had managed to attract 80 million or 90 million members in the US.

Wednesday's letter to shareholders also mentioned that the company's smart speaker products have also been selling well. "2017 was our best year yet for hardware sales. Customers bought tens of millions of Echo devices," Bezos said.

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