I think what Oculus did was massively de-risk their next ten years. In their business plans, there were undoubtedly very large gray circles with things like "gain overwhelming market share (how?)" and lists of competitors that included, well, everyone in tech.
If you had to pick a tech company to leverage your disruptive new consumer technology, and make sure it kept the 'awesome', and was hacker friendly, who would you go to?
In exchange, they probably lopped another 10-50x of potential value off the top of their company. Not that they won't see some of that realized in Facebook if this is succesful -- they will. But, they will be sharing the gain around in exchange for getting access to that giant global customer base and being able to tap the considerable resources available at FB.