Pandora Media Inc. filed an S-1 form with the SEC to raise $100 million in an initial public offering.

The reasoning for the $100 million IPO? Pandora says it has 50 percent share of all Internet radio listening time among top 20 stations and networks in the United States, and it also claims 80 million registered users of its Pandora Internet radio service.

The Pandora app is a top five most used app across all four major smartphone platforms in the United States, according to a September 2010 report by Nielsen. According to Apple, in January 2011, the Pandora app was the No. 2 all-time downloaded free iPhone app and the No. 1 all-time downloaded free iPad app.

Pandora generates its revenues through advertising and keeps its radio service free. About one minute of audio ads occur per hour of listening and display and video advertising occurs when users interact with Pandora.

In fiscal 2010, Pandora had $55.2 million in revenue, which grew to revenues of $90.1 million in only nine months ending Oct. 31, 2010.