I think this line of reasoning depends on whether there are actual previous examples of 'gimmicks' the size of facebook evaporating in this manner. Without any research on the matter, I'm inclined to say Facebook is simply too large and to entrenched at this point to be a fad. It's qualitatively different from fads in the same arena that came before it.

But it's an honest question. If there are previous examples of facebook-like fads suddenly disappearing, the line of thinking is sound and possible.