Google and Yahoo are both planning to turn their e-mail systems and personalized home page services into social networks. But, as Saul Hansell writes in the NY Times Bits blog, "Running social networks is like starting nightclubs. You need music and beer, of course, and some hard-to describe magic that draws people to the club. Yahoo and Google are so big that the most they could aspire to would be to create the online equivalent of a crowded bar at a train station, rather than a club for a particular sort of person."
So, what exactly is that "hard-to describe magic" that will launch the next music social network? I don't think it's online beer sales and I don't think anyone comes away from their MySpace experience with a feeling of hard-to describe magic...