2. Median employee salary is wrong, you should look at average salary if you are comparing it to an average (revenue) figure.
3. You should correct for benefits and taxes. I don't know what they are, but salary is only part of the cost of an employee.
I know you are trying to make a quick point, but FB has margins ~40% and all-in employee costs can be 50% higher than salary. Assuming the median comp is close to average, then it's possible FB is making appr. $700K or so per employee in profit and paying out appr. $300K which doesn't sound quite as bad.