hello!
firstly, you have to know the IP addresses of the social sites what you want to block. you can look up those with dig (or nslookup).
for example, we want to block facebook.com:
# dig facebook.com +short
69.63.189.16
69.63.189.11
69.63.181.12
now we see that facebook.com has 3 IP addresses.
we can block them simply with these commands:
# iptables -A INPUT -s 69.63.189.16 -j DROP
# iptables -A INPUT -s 69.63.189.11 -j DROP
# iptables -A INPUT -s 69.63.181.12 -j DROP
of course you replace the IPs if you want to block something else
note that you need root access to use iptables.
hope it helps
