CBS has unearthed a novel idea for growing the U.S. audience of Last.fm, the U.K.-based music website cum social network it purchased last May. It's going to try promoting it. In the Stateside push, the network is offering free, streaming music from the four major music labels and a bevy of independent artists. The ad-supported network bills itself as "redesigning the online music economy": It will pay artists per play. That explains why it's agreed to all sorts of icky streaming restrictions, such as limiting songs to three plays per user. It's a mutually agreeable term where CBS doesn't end up pissing off the music industry, which provide the songs it plays on its many conventional radio stations, nor does the music industry end up slighting CBS, an important promotional partner. Lost in all the tradeoffs: The interests of the very listeners CBS hopes to attract.