Pandora

Subscription-based and free Internet radio services like Pandora and Spotify accounted for 23 percent of average weekly music listening time among U.S. consumers between the ages of 13 and 35 during the fourth quarter of 2012, up from 17 percent in the year-ago period thanks to increasing mobile engagement, according to a new NPD Group report.

Pandora Media is extending its digital music platform to devices running Microsoft's Windows Phone 8, pledging an uncapped mobile listening experience throughout the remainder of this year.

Amazon's is the latest digital media giant reportedly considering a streaming music service, following speculation that both Apple and Google are both readying efforts to challenge established players like Spotify, Pandora and Rhapsody.

Pandora Media reported fiscal fourth-quarter revenues of $125.1 million, a 54 percent year-over-year increase fueled by mobile growth. The streaming radio company also announced the resignation of longtime Chairman and CEO Joe Kennedy.

If it seems like there are already too many streaming music services to count, just wait until Apple and Google enter the mix. Neither mobile giant has officially confirmed plans to launch a streaming service, but both seem like foregone conclusions to challenge established players like Spotify, Pandora and Rhapsody.

Google is reportedly developing a subscription-based streaming radio service optimized for mobile devices, taking aim at rival efforts from Spotify, Rhapsody and Pandora.