Amazon's Alexa had a breakout holiday — people even used Echoes to buy more Echoes

Key Points
  • Amazon sold "millions" more Amazon devices than a year ago over the holidays.
  • Echo Dot was the top-selling product on Amazon across all categories.
  • Echo Spot and Echo Buttons also sold out.
Echo Dot and price wars help catapult Amazon as this year's leader in retail

Looking for an indicator that Amazon's Alexa is a beloved product? This holiday season, shoppers asked Alexa to order ... more Alexa devices.

The company said on Tuesday that it sold "tens of millions" of devices powered by its smart voice assistant.

While there are scores of purchases that can be made using Alexa, the top purchases made through "voice shopping" were the Echo Dot and Fire TV Stick with Alexa Voice Remote, the company said.

Amazon's Alexa app also topped the App Store charts for Apple's iPhone, presumably as people set up their new devices.

The small, inexpensive Echo Dot was the top-selling product on Amazon across all categories, and the newer Echo Spot and Echo Buttons also sold out.

Alexa also seems to be a big driver of the Fire TV category. Twice as many Fire TV Sticks (the less-sophisticated version) were purchased this year — but usage of Alexa on the higher-end Fire TV devices was 889 percent year over year in the U.S.

Alexa's breakout holiday seems to come hand-in-hand with a surge in smart-home interest in general: Smart plugs, smart light bulbs, and robot vacuums were also top sellers.

"The more that Amazon can not just get its Echo devices out there, but get its Alexa technology embedded in a variety of things, including cars, that'll be the long-term win," RBC analyst Mark Mahaney told CNBC's "Squawk Alley" on Tuesday.