Apple Inc.'s (AAPL) Siri natural language technology has been a leader among digital voice assistants since 2011, when it was first integrated into the iPhone to bring artificial intelligence to users to answer questions, make recommendations and perform actions. Rivals, though, are catching up fast.

According to a new report from research firm Verto Analytics, the number of Siri users fell in the 12 months through May 2017, to 34.1 million monthly active users from 41.4 million.

Meanwhile, Amazon.com, Inc.'s (AMZN) best-selling Alexa voice assistant more than tripled its user base to 2.6 million monthly users from 800,000. Microsoft Corporation's (MSFT) Cortana was the third most-popular voice assistant with 700,000 users, up from 200,000 during the same period. (See also: Amazon to Control 70% of the Voice-Activated Speaker Market This Year.)

Engagement rates -- which Verto Analytics defines as a "stickiness" factor that compares daily users to monthly users -- for Apple's Siri also dropped to 11% from 22%. During the same period, Amazon's Alexa and Microsoft's Cortana doubled and tripled their engagement rates to 22% and 60%, respectively. Siri'saverage session duration increased, with the overall time that users spent interacting with the device per session expanding from two minutes and 53 seconds to two minutes and 59 seconds. Amazon's Alexa saw a decrease in the time that users spend interacting with it, slipping from one minute and ten seconds to one minute and seven seconds.

Verto found that usage of voice-based personal assistants has increased even as smartphone-based personal assistants decline in popularity. This should be good news for Apple as it sets out to tap the market for home assistants with its new HomePod device. Amazon's family of Echo devices are the leaders in this category. While it has yet to release a smart speaker/home assistant, Microsoft's Cortana is used mainly across a number of its devices, such as its Surface tablets. (See also: Microsoft Is Adding Cortana to Its Office Suite.)

In addition, Alphabet Inc. (GOOG​) subsidiary Google has made artificial intelligence a centerpiece of its future business strategy by incorporating it across products offered by the company, from cloud solutions to home assistants to its search algorithms. (See also: Google Doubles Down On Its AI Assistant.)

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