Extraído do meu arquivo de configuração:
# When is Facebook allowed?
acl allowfacebooktime time MTWHF 12:15-13:45
# Facebook ACL
acl facebookdotcom dstdomain .facebook.com
# Only allow Facebook as described by allowfacebooktime
http_access allow facebookdotcom allowfacebooktime
# Else block facebook
http_access deny facebookdotcom