What happens is - the school or work place admins enter facebooks domain into the blocked list of the firewall configurations and when you try to navigate to facebook all you will see is an error
Here I will teach you how to unblock facebook in just 2 steps:
1. Navigate to one of the Facebook Proxys
2. Enter www.facebook.com in the URL Box inside the proxy site (not the address bar of the browser) and click "Browser" button in the webpage
Enjoy ;)