Earlier quoted context omitted.
Facebook is the largest seller of online display advertising in the world (bigger than Google), and they've still got a relatively immature advertising platform.
Hard drives are also commodities, which means they sell near cost. If Samsung charges $10 more, people buy from another maker. Facebook is almost the exact opposite: its userbase means that, right now, there is no homologous replacement (including Google+, because they don't have the users). Which doesn't mean FB is "worth" what it's worth—to me it seems overvalued, but then again no one is holding a gun to my head a…
That's a pretty gutsy assumption, considering Facebook could plausibly be seriously damaged by something as simple as kids deciding they don't want to hang out at the same place their parents do.