Share Based Payments - Basic Accounting - not a Fiddle
Basic accounting I'm afraid. IFRS 2, Share-based Payment requires compliant companies to measure the fair value of the employee stock options granted to employees and to recognise this amount as an expense in the year in which options are granted.
The valuation of an option, as a "one way bet" is based on the amount of the option and also the volatility of the share price. The infanous "Black Scholes" model is often used for that.
Of course there is no "real expense" but the idea is that companies that do cash compensation should not be disadvantaged.
On the basis that Facebook will have had a generous share option scheme and the share price is volatile, the "Share Based Payments" will have been high value.
Note that if the options are exercised there will be either income tax or capital gains tax paid by the individuals and potentially NI paid by the company - but thats if the shares become more valuabe than the price they were granted at.