Mobile users are increasingly paying for their favorite music service using in-app purchases. For the first time, streaming music apps by Pandora, Spotify and Apple ranked among the leaders in app revenue in August. According to figures released Wednesday by App Annie, a San Francisco-based firm that tracks the apps of numerous platforms, the three music companies landed in the top 10 of all non-game apps.

Pandora ranked No. 2 on both the iOS and Google Play charts even though its popularity -- over 250 million registered users and over 76 million monthly listeners -- comes mostly from the United States although the service is also available in Australia and New Zealand. The company also ranked at No. 2 in July but gained three spots from No. 5 at Google Play.

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The Internet radio leader has had previous success from in-app subscription purchases. In the quarter ended Oct. 31, in-app subscription purchases were a "significant" driver in subscription growth, according to CFO Mike Herring. Earlier in the year, in-app purchase revenue rose after Pandora implemented temporary mobile listening caps.

Spotify moved up to No. 7 on the iOS chart from No. 14 a month earlier. The company began selling in-app subscriptions at the end of June. It charges more for in-app subscriptions to help offset the 30-percent fee payable to Apple. Subscribers can switch to the standard pricing via credit card billing through Spotify's Web site.

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Apple, behind the performance of the Beats Music app, jumped 2 spots to No. 9 on the iOS chart. App Annie explains that in August Beats Music received prominent placement within Apple's App Store. The app was featured in "Apple Apps," a list of apps shown to owners of new iOS devices, and "Apps Made by Apple" in the App Store's "Featured" page. Beats Music added in-app purchases in April, over a month before it was acquired by Apple.

Another mover in August was Free Music Download, an app released at the end of July that was the eighth-most downloaded iOS app in August. Free Music Download allows users to stream and download music from SoundCloud to an iPhone, iPod or iPad. Only tracks designated as downloadable at SoundCloud is available for downloading on the app.