Audio identification app Shazam announced Monday it has surpassed 250 million users worldwide and has expanded its Shazam for TV service to every channel in the U.S.

With one quarter billion users, Shazam is easily one of the most common and most important music discovery tools in the world. The app listens to music — on the radio, on TV, in a club or other social settings — and finds out the artist and track name. In this era of licensed music for commercials and TV programs, music fans benefit from having an app like Shazam — or its main competitor, SoundHound — at their disposal. After all, nobody ever became a fan of an artist they didn’t know by name.

Although Shazam started out as a music-oriented app, it probably wouldn’t have grown from 75 million users in May 2010 to 250 million today without its TV strategy. Shazam began working with advertisers and TV producers to give users of its Shazam app to acquire extra content. Back in February 2011, Shazam partnered with Old Navy to allow viewers of its TV ads to unlock special deals and content. “American Idol” episodes were Shazam-enabled this year.

Using Shazam with TV is old hat now. Shazam’s “TV companion app” works on any TV show in the U.S. The app returns featured music, cast information, trivia and the ability to share a comment on a show or episode on Facebook or Twitter.

That, in turn, can be good for the music business. More Shazam users means more music discovery and more music purchases. [Allindstrom]