have you tried to block facebook via iptables (-m string):
http://blog.mynux.net/2011/08/iptabl...-facebook.html

Why you dont try making an alias and block access to it? I did in this way on pfsense box and it works, think can work in linux too, but if you block it with alias non will be aviable to access facebook, even if you have PC's wich are not filtered in proxy.

Block Facebook with iptables
 

Let's say your LAN IP range is 10.1.1.0/24 then:


### Block connections to Facebook #########################################################################

IPT="/sbin/iptables"

$IPT -A OUTPUT -p tcp -m string --string "facebook.com" --algo kmp -j DROP
$IPT -A FORWARD -d facebook.com -j DROP

echo "Obtaining Facebook IP ranges from whois..."

RESULT=$( /usr/bin/whois -h whois.radb.net '!gAS32934' | tr ' ' '\n' | sort -n -k1,1 -k2,2 -k3,3 -k4,4 )

arr=$(echo $RESULT | tr " " "\n")

if [ -f facebookips ] ; then
rm -f facebookips
fi

for x in $arr
do
echo "$x" >> facebookips
done

tail -n +6 facebookips > tmp.tmp
mv tmp.tmp facebookips

y=0

while read line
do
(( y++ ))
$IPT -I FORWARD -s 10.1.1.0/24 -d $line -j DROP
$IPT -A INPUT -p tcp -d $line --dport 443 -j REJECT
done <facebookips

echo "$y Facebook IP ranges blocked."

#################################################################################################### #######