Cisco used to - and may still - have a policy of laying off the bottom 10% of employees every year, including during hiring freezes. Bizarrely, that meant being rated "excellent" or "exceeds expectations" often meant, "Update Your Resume'" on Monster.com and start emailing it other companies." Cisco is one of a very few companies I won't consider during a job search (unless it's Desperation Station time).
Treating employees like expendable crap will expend employees, including some of the best. If Zucks doesn't know this, it may be taught to him by some of FB's best employees.