I have a similar tic. I just edit my hosts file to redirect those requests to localhost. Then when I mindlessly navigate to one of those sites it doesn't work and I realize I did it unconsciously. I do the same for gmail because I have the same tic for checking my mail.
If I actually want to go to one of those sites I just edit the hosts file and comment out the redirect line.
On my mac, the /etc/hosts file looks like:
# #
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1 localhost
255.255.255.255 broadcasthost
::1 localhost
fe80::1%lo0 localhost
127.0.0.1 news.ycombinator.com twitter.com www.facebook.com reddit.com
127.0.0.1 mail.google.com
If I actually want to go to one of those sites I just edit the hosts file and comment out the redirect line.
On my mac, the /etc/hosts file looks like: