History is against the endurance of Facebook and Twitter, as it is against all the networks that came before and proclaimed themselves invulnerable: the telegraph, the fax machine, America Online, Prodigy, and so on. As people become dimly aware of the walled gardens being erected around their data and increasingly irritated by a culture of oversharing, new networks more conducive to selectively sharing information will emerge. Furthermore, users will demand not just greater control but also greater portability of their data, and if they don't get it they will use tools to extract it.
Ultimately, these newer networks will become a hybrid of three different applications: a three-way cross between a virtual drive, a photo/video/media album, and a location-sensitive status/update feed.
With the availability of these new services and with a rising cognizance of the problems of handing all your data to a single entity, Facebook will lose its coveted status.
Challenge John Feminella to a bet on this prediction!