America's favorite boy genius, Facebook founder Mark Zuckerberg, has announced a new form of messaging. E-mail, the last Internet link to traditional, epistolary, interpersonal communication, is, he said, outmoded. Young people, by which he meant younger than his own 26 years, desired something more nimble for their iPads, mobile phones and other devices. What he proposed was a "social inbox" where users could readily access messages from friends and then sort them—sort of a cross between instant messaging and Twitter.