Do you want it in Squid or on each machine's packet filter? You can find out Faecebook's network:
Code:
host facebook.com
whois -h whois.radb.net 31.13.72.36
Then you can extract all the networks
Code:
whois -h whois.radb.net '!gAS32934' \
| tr ' ' '\n' \
| sed '/^[0-9]/!d' \
| sort -t . -k1,1n -k2,2n -k3,3n -k4,4n \
> /tmp/faecebook.ips
and add them to your Squid configuration or iptables. That's for the IPv4 networks only though.
Which version of Squid do you have?