The guitar strums and a male voice intones: “I always tell myself, just a half hour. But six hours later I’m still on.” The chorus kicks in: “MySpace is the place where I can go to be free. MySpace is the place where I can go to read about me . . . “

The song is a satirical rock track written by a Los Angeles band called The Fresh which, like many others, uses the hugely popular social networking site to promote its music. But it is an anthem that rings uncomfortably in the ears of many advertising executives as they confront perhaps the biggest challenge to the industry since radio gave way to television half a century ago.