Embrace extend extinguish. Valve is not going to block the rift but they will make it's users second class citizens, until VR hardware goes the way of monitors and sound cards. Facebook adds little to this as they don't know gaming and don't have a history of keep multiple businesses going at the same time. So really it's the same company that originally developed the hardware with access to more capital and the risk of being shut down.

In the end VR is going to be on a tiny fraction of computers for a long time. There will be waves of improvements and a few hype cycles (3D TV anyone?)

PS: Sure this time it's different, and next time it will also be different, and the time after that...