Federico Viticci commenting on the WhatsApp numbers:

Free, native and integrated beats “free and third-party” any time for the majority of users when it comes to iOS-to-iOS communication. And it’s not like iOS-to-iOS messaging is a rare scenario nowadays, with over 250 million iOS devices out there and quite possibly a large percentage of them being iOS 5-enabled (iOS 5 runs on the older iPhone 3GS, iPod touch 3rd and 4th generation). iMessage works on the iPhone, iPad and iPod touch. It’s free and supports text and media.

You also have to remember that iOS users (on average) download a lot more apps than other users on other platforms — meaning I would guess the majority of Whatsapp users are iOS users.