The main potential savings is not from avoiding office leases, it's from hiring engineers for 1/3 of the cost or less from places like Europe, India, and Brazil.
Facebook has offices in Brazil: https://www.facebook.com/careers/locations/saopaulo/?p[offic...
Why didn’t they close down the Palo Alto office already?
As a data point, I turned down an offer for FB Palo Alto, one of the main reasons being having to relocate from Toronto. They have offices in Toronto, but no engineering.
Bottom line, they might have some presence in some major cities, but if they're serious about remote work the talent pool definitely grows significantly.
Companies have been forced into large-scale WFH experiments. If the experiments show that productivity is totally fine without open office plans and in-person collaboration over lunch break, then they will inevitably ask why they shouldn't save a ton of money by hiring a greater proportion of engineers outside of the Bay Area.