I would think that this is their attempt to kill the very idea of e-mail. In their world, everybody has Facebook, period.
"We don't think modern messaging system will be email," he noted, saying that he believes seven characteristics that we will come to define "next generation" messaging: seamless, informal, immediate, personal, simple, minimal, and short.
(http://www.huffingtonpost.com/2010/11/15/facebook-email-addr...)
He hasn't given up his dream of becoming the leader in mobile, seamless messaging/communication.
Just put your phone in your pocket when you want to stop replying immediately!
Why!? I compose long emails for a reason! I can't get my point across in short messages sometimes! I might be explaining a long and complex story with some backstory!