Google Glass's proposition is fundamentally "wearable Facebook". I don't think this is a defensive move; it's a "Oh, it's obvious in hindsight; why aren't we doing that, too?" response. Oculus Rift wasn't competing against it, but a Facebook-owned Oculus Rift is a lot more likely to.

In effect, Facebook is asking for the second mover position. There's an old saw about how the first mover in a new market actually fails because he runs into all the problems, but the second mover succeeds because he learns from the first mover's mistakes. That could easily be happening here.