Why blocking sites using Hosts file isn't working?

Question

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. 

Answer 1

Are you sure your entry was in fact saved to the hosts file?  The hosts file on my computer blocks unwanted sites.
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Yes
This is a copy paste of CMD.exe. It shows what's in the host files. 
Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.
C:\Users\mehdi>cd C:\windows\system32\drivers\etc
C:\Windows\System32\drivers\etc>dir \a-d
 Volume in drive C is Gateway
 Volume Serial Number is B291-A8C6
 Directory of C:\
File Not Found
C:\Windows\System32\drivers\etc>dir /a-d
 Volume in drive C is Gateway
 Volume Serial Number is B291-A8C6
 Directory of C:\Windows\System32\drivers\etc
10/23/2010  01:06 PM               846 hosts
06/10/2009  02:00 PM             3,683 lmhosts.sam
06/10/2009  02:00 PM               407 networks
06/10/2009  02:00 PM             1,358 protocol
06/10/2009  02:00 PM            17,463 services
               5 File(s)         23,757 bytes
               0 Dir(s)  936,919,461,888 bytes free
C:\Windows\System32\drivers\etc>type hosts
# Copyright (c) 1993-2009 Microsoft Corp.
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
# For example:
#      102.54.94.97     rhino.acme.com          # source server
#       38.25.63.10     x.acme.com              # x client host
# localhost name resolution is handled within DNS itself.
#       127.0.0.1       localhost
#       ::1             localhost
127.0.0.1 facebook.com
C:\Windows\System32\drivers\etc>