So we basically are seeing that google was "right" from their point of view in securing a mobile OS. Facebook maybe thought this day would never arrive and could have avoided that by doing their own mobile OS too. Well, maybe they declared this battle as lost and focused on the next one: VR. Problem is that VR took too long to arrive and death on mobile can kill Facebook?
VR's probably not going to be bigger or even equivalent to mobile, so they should've tried harder.
Ubiquitous computing may be the next step from there. Something like “the world is the computer and you just interact with it”.