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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).

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...