"Fuck you" money, basically. The difference between $FU and 2 x $FU is pretty insignificant, so haggling past that point is irrational, but the difference between 10 x $FU and 0 - huge lawyer fees is enormous and the probability of coming up worse than empty way too high. In his position, I'd take a "large" cash (or easily-liquidated) settlement over almost anything available via litigation, even seemingly enormous portions of the company.

Of course, I also don't think Facebook's star is going to exponentially rise over the next three or four years, so I may not be in the majority here.