I read today that the average tenure at Facebook was just over 2 years, and that's actually longer than other FAANG companies. That seems crazy as I hear about people spending over a year just preparing to get in. Also, as stock options vest over 4 years, why would someone leave before that? On the other hand, if the stock options are fully vested after 4 years, what would lead someone to continue to work at the company considering their overall compensation would then be dropping dramatically.