I don't know and they don't disclose it. It's the vast majority of what they pay for physical security, building maintenance, cleaning, hospitality, etc. This happened across all industries. A few decades ago, janitors and cleaners worked for the company, now they're outsourced to keep them out of scope for unions and to be more "flexible" (i.e. hire and fire at will without negative press).
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.
The NYT has a great article on that, not related to Facebook but it shows the general trend: https://www.nytimes.com/2017/09/03/upshot/to-understand-risi...