The best theory I've seen is that Zuckerberg was looking for an angle where Facebook could control a platform, rather than simply be a program on somebody else's. Look around, see VR, and think "This space is in its infancy, and doesn't currently have a market-dominant platform" and then bet big.

Because at the end of the day, Facebook didn't have much to lose. The social media wars were over, FB and especially Instagram had won. As long as they didn't actively break anything, the information->ads->money pipeline will keep flowing until the entire space gets disrupted by something fundamental shifting under their feet.

A VR-2ndLife that actually caught on could honestly be a relatively believable such shift. Ultimately it didn't, but that's still alright news for FB: no big changes means the gravy train is still rolling.