Fans of Sirius XM Radio (NASDAQ:SIRI) have typically dismissed the surprising success of Clear Channel's (NASDAQOTH:CCMO) iHeartRadio, and rightfully so. 

Sirius XM has been able to grow its user base every single quarter since the launch of the terrestrial radio giant's streaming app, closing out its latest quarter with more than 25 million subscribers. 

A common knock for iHeartRadio is that it's primarily a music platform, despite the presence of comedy channels. However, Clear Channel is making things interesting today by introducing talk content.

Magnetic and sometimes polarizing radio icons Glenn Beck, Sean Hannity, Rush Limbaugh, and Ryan Seacrest will be available. In an intriguing twist, you may be available, too. The new digital platform will be opened up for anyone angling to make a podcast.

We'll see how this plays out with Sirius XM, which itself reports quarterly results tomorrow. 

Sirius XM has been able to withstand the onslaught of free streaming apps before, but as the competition broadens its offering, it may limit the pricing power of the premium satellite radio platform.

Sirius XM has still gone on to hit new five-year highs this month, shaking off the challenge posed by Pandora's (NYSE:P) growth and the upcoming rollout of Apple's (NASDAQ:AAPL) iTunes Radio.

Given Sirius XM's scalable model and high fixed costs, every incremental subscriber is worth more than the one that came before. Slow growth is fine. Stalling or potentially going backwards is something else entirely.

Investors may want to monitor the success that iHeartRadio has here. The ability to tune into hundreds of terrestrial stations online was a novelty at first, but the application raised its game two years ago by introducing Pandora-like music discovery features. It's the same platform that Apple is gunning for with iTunes Radio later this year.

Now iHeartRadio is becoming more like the talk radio that many associate with the listenership experience. Sirius XM still has its proprietary content that's worthy of a premium, but it can't ignore the changing marketplace.  

Longtime Fool contributor Rick Munarriz has no position in any stocks mentioned. The Motley Fool recommends Pandora Media. It recommends and owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.