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.
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.