Loans based on assets like stocks/bonds/other assets with highly variable prices always have collateral requirements. If the loan is backed by 100M in facebook shares and the price of stock drops in half you will have to hand over more stock for collateral. If the price doubles, you can ask for your collateral back.

It is doubtful Z has any margin call issues as he has so much stock, I can't imagine he would have pledged even 5% of it for loans, so he can just hand them another chunk without even blinking (which he generally doesn't do any way)