Nearly every new mobile music service is hailed as a "Pandora killer," yet the leader in Internet radio continues to defy expectations. Pandora has continued to steadily grow after Apple and Spotify made strong moves into mobile streaming music.

Since iTunes Radio launched in September, Pandora listener hours are up about 27%. And since Spotify debuted a free mobile service -- not on-demand, but more than personalized radio -- in December, Pandora's listener hours have risen about 16%. Growth in Pandora's monthly active users grew at mid-single digit rates. Interpretation: fewer new listeners are coming in but the average listener is staying longer.

A few caveats should be noted. Pandora has not yet converted its huge listenership into a profit. The company's business model still has its share of critics both on and off Wall Street. Over the long term -- think two or more years -- Apple, Spotify and other music streaming services could improve their "lean back" products and eat into Pandora's growth and market share. Uncertainty remains about the direction music licensing rates will go in the coming years.

Setting aside the financial picture, Pandora continues to fair well in a crowded marketplace. Recent trends suggest Pandora and Spotify, which compete for listeners in very different ways, are coexisting well. We know that two important Pandora metrics, listener hours and average listener hours per user, have risen since December. And from Spotify CEO Daniel Ek we know the free mobile service has resulted in "huge uptake" in paid subscriptions. The two services don't appear to be stepping on each other's toes.

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Spotify and iTunes Radio will need to grow before a competitive threat to Pandora can become clear. Spotify has three million subscribers and an unknown number of active users in the United States. If the ratio of global subscribers/active users applies to the United States, there are 12 million Spotify users in the country. iTunes Radio has about 20 million, according to Edison Research users. (Apple SVP Eddy Cue said last month the service has 40 million listeners, although it's possible that number represents registered users rather than monthly users.) Both are a far cry from Pandora's 77 million active users.

The takeaway is clear: Consumers are not yet attracted to a multi-purpose digital music service. They are content with Internet radio services that focus on Internet radio and on-demand services that focus on being on-demand services.