Spotify [3] is challenging Pandora [4] on home turf, finally launching a radio service for free for its iOS users in the U.S. Music on mobile was only available for Spotify's Premium subscribers (who fork over $10 a month), but Spotify's new service is free. AllThingsD [5] picked up on a few characteristics of Spotify's radio service that were a little different from the way Pandora does things: For one, Spotify relies on a social graph [6] (and the tastes of humans) to line up selections similar to a song or theme you picked. Pandora's method, on the other hand, is to use similarities in the musical structure of the song or band at hand.

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