Why blocking sites using Hosts file isn't working?

Hello 

I need to block a few websites on my home computer. Did a quick search and found out I had to work with the hosts files. So opened up Notepad, ran it as Admin and then opened Hosts and at the bottom added ' 127.0.0.1 facebook.com' then saved the file. But facebook is still opening when I go to the page. 

can someone tell me why  using the Hosts file isn't working?

I did read somewhere that it might have something to do with security features on my system. FYI, I m using a trial version of Norton Anti Virus and I am running a Windows 7 machine. 

Thanks. Any help would be appreciated.