Facebook has implemented HSTS preloading that instructs a browser to always use SSL/TLS to communicate with eligible websites. The modification aims to improve security and navigation speed for Facebook and Instagram links. Facebook determines the links that are eligible for HTTPS based on two sources: The Chromium preload list, that is currently used in most major browsers;Recording HSTs headers from sites shared on Facebook, in this case, the browser preload lists is updated with any sites that serve HstS.”]
Source: https://securityaffairs.co/wordpress/69916/social-networks/facebook-hsts-preloading.html