This is happening because you have configured filters based on keywords in the URL. When accessing https://www.facebook.com, an encrypted connection is established with facebook's servers and your router cannot inspect the URL anymore.
You should try blocking facebook's IP range
Following IP address ranges are known to be used by Facebook:
184.108.40.206 - 220.127.116.11
18.104.22.168 - 22.214.171.124
126.96.36.199 - 188.8.131.52
184.108.40.206 - 220.127.116.11
Additional IP ranges which may be assigned to Facebook Inc.
IP list from here
If you are able to change your infrastructure, then you can consider using a web proxy or a SSL proxy as GdD pointed out.