Its not that weird. They had 200M in revenue last quarter[1] with about 2000 employees. That is about $100K/employee, since employees probably cost more than that at the median, its not really a sustainable strategy. Swap out employees who are making more $/employee to boost the average and maybe you have a going concern.
[1] http://investor.zynga.com/releasedetail.cfm?ReleaseID=800274