Please try to create a deny firewall policy for Facebook services as suggested by AEK. If that does not work, try to sniff traffic or investigate if your clients are using ECH.
Blocking Facebook with Web Filtering This recipe explains how to use a static URL filter to block access to Facebook and its subdomains. By using SSL inspection, you ensure that Facebook and its subdomains are also blocked when accessed through HTTPS.
I been testing to do URL filtering with whitelist approach (allow some URL and block everything) and also create SSL decrypt policy. As I was checking, it seems like not working to block facebook.
How to create alias and block Facebook traffic (IP Addresses and HTTP/HTTPS URL of Facebook) using pfSense firewall rules. Following steps are useful with sites such as Facebook that consume large amounts of IP range but are constrained within a few net b
APP Enforcement provides a simple and fast solution to block both the Facebook page and the Facebook App. This article is going to demonstrate how to configure it.