I've made that same argument, but... how did Facebook get its initial critical mass of users if it provides nothing new? There must be some reason all those college students signed up for it.

Facebook initially offered a more socially exclusive version of MySpace (i.e. college students). By doing so they were able to tap into established social networks - e.g. campus organizations such as clubs, fraternities and sororities as well as the general sense of community created by the US college system through inter-collegiate athletics.