> There's no reasonable way to say FB should have less engineers without being a high level director of the company
I'm going to assume you don't literally mean only company directors can levy logical criticisms of a company.
Consider the fact that the FB app itself was built with a very small team to begin with and only once it became 80-90% feature-complete did the exponential growth in engineers suddenly become "needed".
I can understand why Microsoft needs tons of engineers. OSes are complicated and have lots of moving parts (drivers, file systems, kernel, etc) that are all absolutely required and must be constantly updated, even without bringing their other products into it. But Facebook? At its core, it's a single webapp that serves simple relatively static pages. Yes, at web scale, so it's not entirely trivial.
My explanation is that Google and Facebook face a similar dilemma: most of their revenue comes from a single, relatively simple core product. To really optimize that core product, they might need a couple of dozen highly skilled engineers, along with a lot of equipment to scale it.
But that's not why they hire thousands. It's because they're both constantly trying to branch out for growth. In terms of revenue it is fair to say that strategy has been a failure for Google and except for a few of the "bells and whistles" FB has added like IM, probably for them as well. This problem is caused by the way that tech companies are primarily valued by perceived growth potential rather than actual earnings. They can get away with hiring lots of useless people because capital costs in this industry are fairly low.