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.
But, you get a couple hundred million users that make next to no revenue and will be incredibly difficult to monetize and your star has risen. Pretty hard to compete with that rock solid proven model I guess.
On the other hand you have a hardware startup with no customers and an unproven product. Sure they have sold a bunch of dev kit's, but those are far from being ready for the consumer market. In addition to not having a customers, they have Sony building a similar product and remember that Sony already has hardware in millions of homes and therefore roll out would be much easier. Occulus might build a great product, but competition is going to be much harder given Sony's pursuit. I would argue that Occulus has proven to be an innovative and smart company, but in no means have they proven to be a successful business and therefore their "star has not risen" just yet. Speculation on future value is much harder with Occulus and has too many "ifs" and therefore, currently, Occulus is not worth anything close to the $19B paid for WhatsApp.