In its first earnings announcement as a publicly traded company, Pandora revealed that its Internet radio service now accounts for approximately 3.6% of all radio listening in the U.S.

That’s based on the roughly 1.8 billion hours its 100+ million users spent listening to the service during the second quarter, which was up 125% from the same period in the previous year. The 3.6% share was double what the company reported during Q2 2010, and up from 2.3% at the start of this year.

Pandora’s usage growth helped fuel strong financial results for the company, which reported second quarter revenue of $67 million, up 117% year-over-year. Of that revenue, $58.3 million came from advertising. Half of that advertising was generated via ads on mobile devices.

Speaking of the latter achievement, CEO Joe Kennedy offered in a statement, “Pandora's mobile advertising revenue for the first time comprised approximately half of total advertising revenue as we lead the way in the nascent but fast growing mobile advertising market. Pandora continues to grow our market share of U.S. radio as we fundamentally transform one of the last forms of traditional media."

Investors seem to be reacting favorably to Pandora’s announcement, which included a revenue forecast of $270 million to $275 million for all of 2011. Shares of the company were up nearly 7% in early trading on the NYSE on Friday morning.