Stock can be used as collateral to get a bank loan (the uber rich use this to avoid capital gains taxes). I would imagine that is what they'll do to generate cash for disbursements from the foundation (they have to pay out at least 5% per year).

I would be shocked to find they sold the shares. I'm sure them keeping them for a long period of time (possibly forever) and Zuck retaining voting power over them was a key part of this transaction.