I'm aware of these concepts, but they don't defeat my objection. The question is still open whether Facebook is in that region or in the "grow by a factor of ten in the next year" region. Hard to predict the answer to that question.
Google was able to maintain growth doubling until 2006. Then the curve started to flatten out. Assuming FB is where Google was in 2003, they can grow. FB right now claims 500million users, but only about Google's 2003 revenue figures. In 2004, Google boasted they had 60million unique visitors. I'll leave the rest of the math up to the reader.