A funny thing happened on the way to Pandora's IPO.

The Internet radio juggernaut is being served with a subpoena in connection with a federal grand jury investigating the app maker's information-sharing practices. Pandora was informed that it's not the specific target of the investigation. It believes that several companies behind popular apps running on Apple's (Nasdaq: AAPL) iOS and Google's (Nasdaq: GOOG) Android platform were served with subpoenas earlier this year.

However, something as subtle as this casts a cloud over the cloud-based music service.

It's a shame because Pandora's growth is undeniable. There are now 82 million registered users, streaming through the music discovery site's catalog of more than 800,000 songs. We're not talking about dormant accounts here. This isn't Latino social networking site QuePasa (Nasdaq: QPSA) with an impressive cumulative registration rate but engagement levels that imply a much smaller base of active users. Pandora served up a whopping 3.9 billion hours of content last year. In other words, the average registered user has tuned in to Pandora between 48 hours (based on the 82 million registered users at the end of the year) and 87 hours (based on the 45 million members when the fiscal year began). It's the equivalent of Sirius XM Radio's (Nasdaq: SIRI) more than 20 million subscribers tuning in for about 200 hours in a year.

Pandora's hoping to be listed on the New York Stock Exchange under the ticker symbol P for Pandora -- but it may as well be P for patience.

Pandora has posted losses in each of the past five years, though its deficits have narrowed over the past two years. Revenue soared 150% to $137.8 million last year, but 87% of that comes from advertising.

Pandora is generating nearly double the $64.5 million that Sirius XM collected in net advertising revenue last year, but the real challenge -- and opportunity -- rests in beefing up its premium subscriber base.

It won't be easy. The Apple and Android app stores are brimming with free audio streaming sites. For every new media upstart incorporating a smartphone pay wall -- as CBS' (NYSE: CBS) last.fm did this year -- you have plenty of terrestrial radio stations and indie broadcasters more than happy to give away their apps for free.

If Pandora can't squeeze more of its users to pay for unlimited ad-free streams, we'll be down to assessing its ability to grow its user base as it tries to milk more money out of its advertisers. There's no evidence that Pandora is peaking on either front, but now it needs the federal grand jury investigation to play itself out and a reasonable IPO valuation to get this party started.  

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Longtime Fool contributor Rick Munarriz is a subscriber to both Sirius and XM. He does not own shares in any of the stocks in this article. He is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.