http://developers.facebook.com/
$ nslookup facebook.com ns1.facebook.com
Server: ns1.facebook.com
Address: 204.74.66.132#53
Name: facebook.com
Address: 69.63.189.11
Name: facebook.com
Address: 69.63.181.12
Name: facebook.com
Address: 69.63.189.16
$ nslookup www.facebook.com ns1.facebook.com
Server: ns1.facebook.com
Address: 204.74.66.132#53
Non-authoritative answer:
*** Can't find www.facebook.com: No answer
(YMMV based on local cache etc.)