Blocking / Allowing Facebook using Microsoft ISA can be difficult. I have created a Domain Name Set for ISA that you can import. They can be found:
ISA 2004 SP3: here (zip file)
ISA 2006 SP1: here (zip file)
Plain Text: here (zip file)
It is very possible there are some non Facebook related websites in this list, so please verify these are suitable for use in your environment. These domains can change over time, so if you find any that aren’t in the list, please let me know.