My guess is that when Oculus Rift found out about Sony coming to market, they knew they needed someone with deep pockets to speed up their implementation.

They were probably in talks with a number of other companies, but Facebook jumped the gun and made an offer they couldn't refuse, in order to prevent their "competitor" from getting it.

</speculation>

I wonder if FB is going to use OR to rebirth Facebook games, or maybe make their own foray into the gaming industry.