/etc/squid/blockedsites.squid

[root@leela ~]# cat /etc/squid/blockedsites.squid
#blocked sites
www.facebook.com
www.gmail.com

/etc/squid/squid.conf

acl Safe_ports port 488 * * * * # gss-http
acl Safe_ports port 591 * * * * # filemaker
acl Safe_ports port 777 * * * * # multiling http
acl CONNECT method CONNECT
# ACL blocksites
acl blocksites dstdomain "/etc/squid/blockedsites.squid"

http_access deny blocksites

# Recommended minimum Access Permission configuration:
#
# Only allow cachemgr access from localhost
http_access allow manager localhost
# Deny access to blocksites ACL
http_access deny blocksites