Carmack is now working for Facebook shareholders. If that doesn't give you pause, I'm not sure what else to say.
Companies buy other ones for the people, yes, but also for the name and intellectual property. This enables them to sell the product to unwitting customers with the name or, failing that, they can prevent competitors from doing it through their trademarks. They maintain the physical talent but I find that the most creative people tend to be the least accepting of this corporate reality. They probably had good intentions with their visions (rather than "selling out") so that likely applies.
Facebook has had a rough patch the last few years. It's nothing new in nature or in companies (I personally liken companies to species in an ecology, evolved to mini-states of their own on the political scene). The old business life-cycle rears its ugly head, but applied to technology companies. They inevitably grow massive, stagnant, and focus more on R&D. R&D is about people and IP. It's about getting somewhere first.
So, that all being said, this is a natural step for a technology company that wishes to survive. The question is whether or not it was the right one - and obviously, that's where the shareholders come into play. People like the idea of VR, and the more knowledgable might even like Carmack, then let the accountants fill out the rest. Have no doubt about it, though, it was an idea sold based on speculation and it's hard to separate oversight from that with conventional corporate structures.