More than 3 million people joined Amazon Prime in the third week of December alone, 200 million items were shipped for free to Prime subscribers, and more than two times as many Amazon devices were sold than last year as part of Amazon's 2015 holiday season, the online retail giant said today.

70 percent of customers shopped on mobile

Christmas Eve became the biggest ever day for Prime Now deliveries, and Prime Video also saw an uptick in usage, doubling the amount of hours streamed during the holiday period in 2014. A record number of movies were watched on the service, with the most viewed — The Hunger Games: Mockingjay Part 1 — appearing to be a victory over major rival Netflix, which lost the US streaming rights to the wildly popular movie series in August. As Netflix has found success with original shows like Orange is the New Black and Jessica Jones, Amazon has followed its path — its original series The Man in the High Castle became the most watched TV season ever on Prime Video.

In addition to showing the growing market penetration of Prime, Amazon's sales data also shows a shift toward mobile. Almost 70 percent of Amazon.com customers shopped on a mobile device during the period, with the number of US-based Prime members using a smartphone to buy items more than doubling from last year's total. The number of customers using Amazon's mobile app also doubled from 2014's total, and on Cyber Monday, the retailer said it was processing 33 electronics orders per second from a mobile device.

Chinese food was popular on Christmas day

Amazon's own devices did well during the holiday period, selling two times the amount they did last year: the company's Fire tablet became Amazon.com's best-selling product, and the Fire TV Stick took third place. Those two products, along with Amazon's Echo speaker, made up three of the top five items ordered on Amazon.com using the Prime Now delivery option on Christmas Eve. Customers buying the Echo used it primarily for its music functions — "play music" was the most common request for the speaker's Alexa assistant, with "White Christmas" one of the most selected holiday songs. When it wasn't being ordered to play music, Echoes across the United States were also being used to order food, with Chinese restaurants getting the most Alexa-enabled custom on Christmas Day itself.

But while Amazon's holiday period broke a number of its sales records, the company doesn't specify exactly how many new Prime members it added to its books during the holiday period. Last year, the company said 10 million tried the service — this time it simply says that Prime membership "continues to grow," and that there are "tens of millions of members" worldwide. The number also doesn't confirm how many of these Prime members are on free trials, and how many are paying customers.

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