Want to block Facebook entirely for yourself or someone you care about? It’s easy.
Instructions for a Mac OS X:
Step 1:
Open up Terminal (It’s in Applications/Utilities) and enter this text and hit enter, followed by your password:
sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit /etc/hosts
This is your hosts file, which is the first place your computer checks for DNS lookups.
Step 2:
Add this line:
127.0.0.1 facebook.com
Once your hosts file looks like this, save it: 
Open Terminal again and insert this: dscacheutil -flushcache. Then restart your browsers and your machine to be certain your DNS cache is flushed down the drain.
On a PC:
1) Right click on Notepad and pick Run as administrator. Click on File then Open then browse to windowssystem32driversetc In the drop down list change Text files (*.txt) to All files (*.*)
2) Open the hosts file and add 127.0.0.1 facebook.com
Reduce those life minutes:
Related reading: NYTimes: “F.T.C. Backs Plan to Honor Privacy of Online Users“
Moneydick 
How do you block it on other systems?
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Replace facebook.com with another domain
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