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 and forcing me to buy its stock. The nice thing about markets is that we can bet with our money, or not.