> Is he going to pay the severances out of pocket?
More or less? The vast majority of his personal net worth is tied up in FB stock.
As to the other questions -- the severance package is pretty generous.
He is not putting the shares down himself. He is just subject to price fluctuations like everyone else — so how is he taking personal responsibility for it?
Layoffs don't necessarily lead to share prices dropping, in the short term or long term. It definitely wouldn't impact the loans he presumably lives off of by using said shares as collateral.