I haven’t studied FB much, but do they have a strategy for continued growth? They don’t seem to be expanding into new growth areas as much as Goog or Amazon?

Look at their revenue and user growth. They’re still growing like a weed.

soon they will saturate world population. Their monthly active user count might even be close to the maximum they can attain even...presumably a lot of the people in developed countries who aren't using facebook now are probably doing it for good reasons and so probably won't add to facebook's MAU count.