I read somewhere that they were talking about using the Facebook app itself to track remote employees whereabouts. Having trouble finding the reference right now though.
Employees who attempt to wiggle around those compensation adjustments will be subject to “severe ramifications,” [Zuckerberg] said, as the company needs to account for employee locations to avoid violating tax laws. Zuckerberg said Facebook will monitor adherence by checking where employees access its VPN. Facebook also uses its own apps' to track employee locations, according to CNBC [...]
(See https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu.... about SNI.)