Facebook hype will fade
facebook growth went something like this:  college --> high school --> young adults --> everyone

Trendy stuff generally follows the same cascade, more or less, where you don't see college students emulating the dress habits of the elderly.facebook's biggest potential for failure is in not capturing the next generation of young users.  The young users pick up some other social network, everyone else follows suit, and facebook withers, slowly starting to resemble an '85 buick.