They have some of the best engineers and VPs in their space, huge hype, mindshare, and by all accounts a kick-ass product. Forget why Facebook is interested, I don't get why Oculus would even consider selling for 10% of a WhatsApp. Their star was rising if anyone's ever was.
WhatsApp was already a star. It had millions of users and was the "go to" app for communication in many countries. We forget that here in NA. Oculus, as cool as it is, still doesn't have a huge customer/user database.