Rules of investing
1) Shares can go down as well as up.
2) Don't invest what you can't afford to lose.
3) If it looks too good to be true, it is.
4) Do your own research, don't rely on those employed by the company
There have been articles for some time questioning the FB IPO valuation, so unless there's some malpractice uncovered in the way the IPO was handled, the investors have no one to blame but themselves if they have lost money.