I think arturadib was talking about the user-base BEING Facebook's product.

You are correct as well, when you say that the user-base is their competitive advantage. It is a competitive advantage in the production of their product... a centralized pool of users.

You are mistaken, however, to categorize the UX as the product. The UX is simply a clever widget used to build the product... again, a centralized pool of users.