Blocking Facebook using /etc/hosts

harrymc is close but for some reason OS X (as of 10.8.2) doesn't respect the IPv6 loopback address of ::1 (probably a bug), so you have to use fe80::1%lo0. The reason you need to block IPv6 is because Facebook will serve you their site over V6 if your ISP supports it. You can easily verify this by installing a browser plugin that displays an icon when a site is being served via IPv6. The reason this wasn't a problem for you before is because Facebook likely only recently started serving their site over IPv6.

So the correct answer is:

# Block Facebook IPv4
127.0.0.1   www.facebook.com
127.0.0.1   facebook.com
127.0.0.1   login.facebook.com
127.0.0.1   www.login.facebook.com
127.0.0.1   fbcdn.net
127.0.0.1   www.fbcdn.net
127.0.0.1   fbcdn.com
127.0.0.1   www.fbcdn.com
127.0.0.1   static.ak.fbcdn.net
127.0.0.1   static.ak.connect.facebook.com
127.0.0.1   connect.facebook.net
127.0.0.1   www.connect.facebook.net
127.0.0.1   apps.facebook.com

# Block Facebook IPv6
fe80::1%lo0 facebook.com
fe80::1%lo0 login.facebook.com
fe80::1%lo0 www.login.facebook.com
fe80::1%lo0 fbcdn.net
fe80::1%lo0 www.fbcdn.net
fe80::1%lo0 fbcdn.com
fe80::1%lo0 www.fbcdn.com
fe80::1%lo0 static.ak.fbcdn.net
fe80::1%lo0 static.ak.connect.facebook.com
fe80::1%lo0 connect.facebook.net
fe80::1%lo0 www.connect.facebook.net
fe80::1%lo0 apps.facebook.com

source: https://superuser.com/questions/544789/why-cant-i-block-facebook-using-etc-hosts-on-mountain-lion-os-x