try to block on CONTENTFILTER and CUSTON BLACKLIST
facebook.com
That's it.
This works. any user can use ip address of the facebook to login.
so i add below ip's to custom blocked list and voila..........it worked. so far i havent found any one who by passed it..
69.63.189.16
69.63.181.12
69.63.189.11
69.63.189.12
69.63.184.142
69.63.187.17
69.63.181.11
69.63.176.13
69.63.181.15
69.63.184.142
69.63.187.17
69.63.187.18
69.63.187.19
69.63.181.11
69.63.181.12