I don't know much about the FB employee stock plan. If they were given grants, the price doesn't matter. They gained shares of the company. If those shares were labeled $25 when they were in fact worth $15, they can't be said to have lost $10 because they actually netted stock worth $15. You might argue taxes as they would have to pay tax on the $25 income, but when they later sell, they can claim the $10 capital loss. Assuming they are in the same tax bracket, it is a wash.

If they were given options which the employees had to pay $25 to exercise, then yes, the employees lost money. However, they had the same opportunity to make the analysis that any other investor had. No one forced them to exercise their options, and those who did paid too much.