To go back to the mobile analogy, I don't think Google chose to not compete on iOS vs the App Store. It's a large possibility that Facebook would be willing to lock down Oculus for more profits and Facebook has shown it wants to play hardball. I don't think developers would be able to ignore the Oculus demographic and they would be forced list their games on the Rift store.
That would kill new Oculus sales. Sure if Facebook gained a monopoly they would do all kinds of shady things, but there is little evidence that's going to happen.
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...
I think valve would argue that if the Rift delivers a second class experience of the SteamVR platform that that is nobody's fault but Oculus's.
So I really don't see how Valve is too concerned about this. If facebook locks down the rift so that it can only play rift games, more fool them. Many people just wont buy a rift.
Even if the Vive fails in the long run, there will likely be many other HMD's that will work with steamVR. Valve can still make a nice profit from all the VR games sold on steam.