Facebook already pays its median employee $240k (https://www.sfchronicle.com/business/networth/article/Yes-me...). You can assume that at those salaries, they've already hired a large percentage of the Bay Area software engineers who meet their bar and are willing to work at Facebook.
So if they want to hire another, say, 5k engineers, they either have to somehow convince 5k people to move to the Bay Area or expand in satellite offices. Word on the street from local recruiters is that you can still get new grads to move to the Bay Area (for a few years at least), but it has become nearly impossible to get senior talent to move in because of cost of living and quality of life issues. So depending on the type of engineer they're looking for, yes, they may be effectively forced to expand in other locations.
So if they want to hire another, say, 5k engineers, they either have to somehow convince 5k people to move to the Bay Area or expand in satellite offices. Word on the street from local recruiters is that you can still get new grads to move to the Bay Area (for a few years at least), but it has become nearly impossible to get senior talent to move in because of cost of living and quality of life issues. So depending on the type of engineer they're looking for, yes, they may be effectively forced to expand in other locations.