Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Pandora Media (NYSE:P) fell nearly 14% Friday after the music streaming specialist announced strong quarterly results, but worried investors with decelerating growth in active listeners.

So what: Specifically, adjusted quarterly revenue climbed 40% year over year to $239.6 million, which translated to adjusted earnings of $0.09 per diluted share. Mobile revenue was particularly encouraging, jumping 50% year over year to $188 million. Analysts, on average, were expecting earnings of just $0.08 per share on sales of $238.5 million.

In addition, Pandora said current quarter revenue is expected to be in the range of $273 million-$278 million, which should result in adjusted earnings per diluted share of $0.17-$0.19. Again, the mid-point of both ranges sits above analysts' expectations for fourth-quarter earnings and revenue of $0.17 per share and $272.8 million, respectively.

However, Pandora also said its number of active listeners as of the end of Q3 grew just 5.2% year over year to 76.5 million -- a tiny sequential increase from 76.4 million last quarter, and a deceleration from the already low 8% and 7.5% active listener growth Pandora achieved in Q1 and Q2, respectively.

Now what: Total listener hours grew 25% to 4.99 billion, which indicates existing users thoroughly enjoy the service and helps explain why Pandora was able to more effectively monetize that base. That doesn't mean Pandora can't continue to do so going forward -- especially as it works to capitalize on its growing opportunity in mobile -- but it can only do so much if growth of its user base stalls. 

Steve Symington has no position in any stocks mentioned. The Motley Fool recommends Pandora Media. The Motley Fool owns shares of Pandora Media. 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.