Blocking facebook integrated with skype

[kalu]

Hello folks
i have successfully blocked  facebook website in my office using squid and squid guard. Now the users are not allowed to browse facebook during office hours.
But some of the smart users are using facebook from the skype (skype has now new feature integrated with skype from which one can post to walls) and since skype is our official chat system we could not block skype, is there any way that i can block only facebook on skype. some layer 7 tricks.
i hope some of you will guide me to the correct path.
thanks
kalu

[Cry Havok]

Duplicate post.