Only you can't short the shares, because they're not free-floating - i.e. you can only buy if you know the right people, so there's none to borrow.
Great article here on it too http://ftalphaville.ft.com/blog/2012/02/01/864331/become-facebooks-friend/
Which includes a number of risks:
"If we fail to retain existing users or add new users, or if our users decrease their level of engagement with Facebook, our revenue, financial results, and business may be significantly harmed."
"Our financial performance has been and will continue to be significantly determined by our success in adding, retaining, and engaging active users. We anticipate that our active user growth rate will decline over time..."