Are you suggesting that Facebook has no performance evaluation process for the employees at all right now? If they do, it seems like you could look at those evaluations and make some decisions immediately. If they don't, then fair enough, implement that. What's the purpose of "turning up the heat" to try to get people to quit? And, as others have mentioned, those who quit first if you make their lives harder will be the ones that have other options (usually the high performers).