The message he wrote, was a message he had to write to keep customers using WhatsApp
WhatsApp has nearly twice the active users as Twitter, and many more than Skype, LinkedIn, Instagram, and even Angry Birds.
Not only that, WhatsApp has a user:employee ratio of over 8,000,000:1. That is quite remarkable, over 8x the users per employee than Youtube.
There is much more value in WhatsApp for FB than just eyeballs.
Facebook owning it means Google, Apple, Microsoft etc don't. Facebook likes that.
Lets remember that as a board member he has a fiduciary duty to FBs shareholders, and that means making sound ($) decisions.
I'm not saying that he would. Or that he could convince enough of the Board that this is the case. But he could.