For comparison Facebook has over 5,000 employees.

Besides programmers there is probably a small army of sys admins to handle their data centers in additional to daily operational IT, business development folks working with partners, the entire ad/sales platform, lawyers and accountants. Plus all of the facility staff for that crazy office (kitchen staff, janitorial, gardeners, etc).

I agree though. That is a lot of people and it's hard to wrap my brain around that since I work at a very small software company of less than 10 people.