I would have to guess that the number of media/content channels, the wide variety of increasingly niche content and the rapid cycling of trends is making the characaturish segmenting by the pseudo social science of demographics less relevant and more absurd.
Take for instance this specimen I spotted at the bookstore today. She was dressed in black and wore suede, rocker-chick books. The book she was holding was Ayn Rand, thick, so it must have been either Atlas Shrugged or The Fountainhead. The magazine she was perusing, Debbli Bliss, a knitting magazine. Any guesses on what she was having for lunch? Who the hell knows. What sort of demo slot does a profile based on that information fit? A liberated, idealistic doer???