Stock options are meant to encourage employees to stay. But Facebook's skyrocketing valuation has created a perverse incentive: It actually encourages employees to quit. That's because ex-employees can sell their shares at any time, while employees have had to seek permission directly from CEO Mark Zuckerberg, who frowns on insiders cashing out. (Never mind that he sold $1 million in shares early in the company's history.) Facebook has created a program that lets employees sell up to 20 percent of their vested shares . A Twitter by Facebook employee Eston Bond asking for advice on selling restricted shares suggests it's already in effect.