In less than five months, Pandora, the premier provider of personalized radio service online, has accomplished what most companies only dream about: becoming one of the most downloaded free applications on the Apple iPhone.

Corroborating this milestone of two million downloads was Apple’s rollout of a “Top Apps” category on December 1, in its new year-end section on the iTunes store. In the free application category that showcases the most downloaded applications of 2008, Pandora took top honors.

For mobile marketers, it’s also a dream come true: a way to reach the most engaged and affluent mobile users in the U.S., and with Pandora’s targeting capabilities, a way to finely tune their ad messages to a rigorously defined audience. What’s also music to mobile marketers’ ears is that the two million application milestone means that Pandora reaches roughly one in every five iPhone users in the U.S. The launch of Pandora’s iPhone application accounts for one-third of all new users since July, and has significantly accelerated the growth of Pandora’s registered user base, on track to reach 20 million before the year’s end.

Currently, Pandora’s iPhone users spend an average of 90 minutes a day interacting with the application, accounting for nearly 1.2 million ad impressions per day. Their average age is 34 and three-quarters of them are male.

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