Web filter works on every page, EXCEPT for Facebook

I had exactly the same problem. It happens because Facebook defaults to HTTPS now. What finally did it was enabling SSL Cert Inspection.  But to counter the fact that SSL inspection caused security errors on all HTTPS sites (even ones in allowed categories), I had to install the FGT SSL Proxy into all the browsers.  Now they can get to the pages they need without panicking about security errors, but Facebook, Youtube, etc are still blocked.

Another thing to be careful of as well - we're using a 90D with about 50 users.  Performance wasn't too bad before this change, but the CPU is running pretty hard now.  So make sure you have enough horsepower for this to work.