I think the shift to VR is actually FB acknowledging they lost the mobile platform battle and can not depend on Apple and Google for its future, so it must move the game on. VR is a platform in which they have a head start and own top to bottom with Oculus.

Essentially Zuckerberg is fast forwarding the future so he’ll have place to sell ads in without a 3rd part like Apple setting the rules.

This is also why he goes full VR and not AR which would likely require a phone somewhere on the side.