Adored by tweens and grandparents alike, Pandora seems to have cracked the code on free music services. The company hired experts who slice apart music into its component sounds as part of what it calls its Music Genome Project. Pandora asks you to name a song or artist, then uses the info to match music that may have similar characteristics. Internet music was always seen as curtains for satellite radio, the inevitable Sirius killer. And Pandora may have delivered. Last year, as Sirius ( SIRI) lost half a million subscribers, Pandora doubled its user rolls to 40 million. The service is Web-based, which means you can log in and hear your so-called stations on any Net device including phones. The Oakland, Calif. outfit raked in a reported $50 million last year in advertising and subscription revenue, managing to swing to a profit at year-end. With a music licensing deal reached a year ago, some of its growth hurdles have been cleared, but giants like Google and Apple also have plans to make it big in Internet music. When was the last time investors got to play music in the key of G for growth?