Cleaning staff, cooks, etc, scale with the size of buildings. Customer support staff, content flag reviewers, etc, scale with the number of users. Since Facebook has so many users, I would expect the second category to be much larger.
Facebook runs a low-support model. At the same time, they offer a lot of perks to their developers. I haven't been to their HQ but from my experience with other companies I would guess that a significant part of the people working there (20-30%+) are working in maintenance, security, hospitality, cleaning, restaurants, etc.
Keeping large offices running smoothly and in nice conditions takes an enormous amount of manual work, a lot of which other employees never see (much of cleaning/maintenance is done over weekends and at night).
Not for large advertisers they don't. Here in Ireland for example their outsourced advertising support & soft-sales, a large part handled by Accenture, are huge. Possibly more numerous than the Facebook EU HQ here. I'm sure the case is similar elsewhere.