Amazon.com said it had its best holiday season yet, having shipped more than 1 billion items through its Prime and Fulfillment services, and receiving a record number of orders for its own Alexa devices.

Sales for Echo speakers based on Alexa's voice-recognition software were nine times more than the 2015 holiday season, Amazon said in a statement Tuesday. The Seattle-based company had trouble keeping them in stock despite "ramped-up production," said Jeff Wilke, chief executive officer worldwide consumer.

Gauging demand for a product is difficult. Retailers risk losing money by overstocking or missing sales and disappointing shoppers by not having enough items available. Amazon actually sold out of its Echo speakers in mid-December. The Echo shortage shows voice-activated assistants are resonating with shoppers. Consumers can use voice commands on the gadget to order pizza, check homework, play music, among other tasks.

"Echo and Echo Dot were the best-selling products across Amazon this year, and we're thrilled that millions of new customers will be introduced to Alexa as a result," Wilke said.

The stock gained 13 percent this year through Friday.

More than 70 percent of Amazon users shopped using a mobile device during the holiday, Amazon said.