Valuation by which criteria? The asking price of an acquisition, paid in RSU with their value measured in the potential asking price for an acquisition? This seems a little circular.
"The fair market value is the price at which the property would change hands between a willing buyer and a willing seller, neither being under any compulsion to buy or to sell and both having reasonable knowledge of relevant facts."
Perhaps a more specific question is: what's the valuation in a real currency such as USD, not the monopoly money of FB RSUs?