Between the period of May 2016 and May 2017, Siri lost 7.3 million monthly users, or about 15% of its total U.S. user base, according to new data released on Tuesday by app-measurement company Verto Analytics. Even those that are still using Siri have cooled to the app, and are now using it less often than they had in the past, Verto said in a statement.

Still, it’s not all bad news for Apple (AAPL): Siri is still the most popular virtual assistant in the U.S., topping Amazon’s Alexa, Microsoft’s (MSFT) Cortana, and various other Android-based assistants. Apple ‘s Siri has 41.4 million monthly unique users in the U.S. It had 48.7 million unique users in May 2016.

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Virtual personal assistants have quickly become critical components in both the mobile and smart home markets. The services allow users to issue voice commands to their respective assistants to create calendar appointments, set timers, and search the Web. The assistants run on everything from smartphones to smart home hubs like Amazon’s (AMZN) Echo.

With such a large iPhone user base in the U.S., it’s perhaps no surprise Apple’s Siri is leading the way. The virtual assistant comes bundled with every Apple device, including the iPhone, iPad, and Macs.

While Apple’s Siri usage is on the decline, Amazon’s Alexa is soaring. The virtual assistant, which runs on smart home devices like the Amazon Echo, had 800,000 U.S. users in May 2016. At the end of May 2017, its user base had grown to 2.6 million. Additionally, Alexa users are proving to be more “sticky” and using the service far more often now than they did last year.

Most other virtual assistants are also seeing their usage climb, due in large part to the number of Android users growing, according to Verto. The researcher didn’t say exactly why Siri’s usage has declined.

Fortune has contacted Apple for comment and will update this article upon response.