shorting means you have to also be able to stomach the upside pain. Maybe it'll end low, but you have to have enough capital to float against margin calls while it climbs. I know people that got eaten alive for shorting yahoo back around 2000 - they were certain it was overvalued and rode short positions from $40 till over $100. Then their wives made them liquidate the positions; if they'd held out a little longer they would have won... shorting isn't all about being right - you also have to have enough capital.

The above quote is very apt. Even if I didn't believe in FB's valuation, I wouldn't short it because I doubt I have the financial fortitude to take the pain.