Is anyone actually more skeptical in the long-term future of Facebook than of any altcoin? (Remember that in order to prevent double-spend attacks, you need to

Is anyone actually more skeptical in the long-term future of Facebook than of any altcoin? (Remember that in order to prevent double-spend attacks, you need to ensure that nobody—and no potential mining pool—is sitting on more computing power than your network has. That's feasible for a single proof-of-work currency; it's technically impossble with even two unless one outcompetes and kills the other, and certainly practically impossible for a new one at this point. And if you're not using proof-of-work, what stability does the coin have once the corporate backer is gone?)