Among the tech giants, Facebook's average revenue per employee is the highest, at $1.16Mn/employee, just marginally ahead of Google's.
So what does a high revenue per employee indicate? Basically, it indicates the revenue generating efficiency of a tech business, where the primary cost driver is really the employees - engineers, managers and sales people. Of course, this is not a perfect measure. Amazon for example, wouldn't technically be as comparable due to their costs of physical goods, but by and large, this works for most tech and knowledge-based companies.
This metric also indicates how much the company can invest in talent. With over $1Mn of revenues per employee, both Facebook and Google can both offer employees better direct compensation. More importantly, they can also invest in top-notch infrastructure, support and perks for each employee. Being able to cream off the best talent in the industry and supporting them well would go a long way in sustaining competitive advantage.
The open question of course, is whether Facebook can scale revenues by10x to Google's $38Bn while maintaining the same employee revenue efficiency, or would it do even better. My guess is that Facebook's revenue per employee would decline from here - mainly because they'll start to employ more people into adjacent areas of growth where monetization won't be as effective as the core business (e.g. a recruiting a full division for Instagram where there's still no revenues...)