My guess is they have right of first refusal for any ex-employees selling shares. So the ex-employees would have to sell to someone sanctioned by Facebook, or

My guess is they have right of first refusal for any ex-employees selling shares. So the ex-employees would have to sell to someone sanctioned by Facebook, or the ex-employees could force Facebook to buy the shares back from them if they found a willing buyer that Facebook didn't approve. With current employees, Facebook probably has a lot more control over who they can sell to. (It may be limited to people who already own shares)