While I think YC is way cooler than Facebook, I suspect from the perspective of Facebook YC is just a little ant below their feet. What incentive do they have to play fair with you?
From across the pond I am willing to assume that you are all really good friends in the valley and that things work out fine. But I also have to admit that Jason might at least have the hint of a point.
Or in other words - YC is about the only entity in this whole game that I personally trust. This is simply the reputation you have built. I don't trust Facebook.
On the other hand, it probably doesn't matter to FB at what stage they see a startup's idea. They'll clone it when it is successful.