I don't know how many people FB actually allocates for their main app, but this reminds me of a chapter in The Mythical Man-Month. It is said over 1000 people and 5000 man-years went into OS/360. I don't see it anywhere today.

Instead, the book proposes The Surgical Team, i.e. about 10 people taking specialized roles, with the system being the product of the mind of a few key people. I wonder how well this aged.