From the WhatsApp's acqusition to closing down Facebook Messages, I think his talk back in 2010 makes sense.

"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.