I'm sure facebook has the answer to that, and it's in the area of several million (depending on your definition of "hardcore")
But you make it sound like selling $300-$500 gaming platforms isn't a lucrative market on it's own, otherwise there wouldn't already be three publicly-traded companies doing just that.
What people fail to understand is that Facebook didn't become a success by being a social networking website, or by collecting user data and selling targeting web ads. There were countless other companies doing those things. Facebook become #1 through it's API. They creating an efficient and extremely extendable one, allowing it to be adapted by third-parties to help spread facebook onto every corner of the web, which in turn brought users back to FB. They're in the API business. So is Occulus. The social networking website and the VR goggles are the front-end revenue generators, but it's their APIs that are responsible for spreading their products outside of their comfortable niches and into widespread adoption.