If it happens that your employer have mandated a policy that Facebook must be blocked on the office, you have no choice but to use proxy or whatever stuffs just to gain access again. On computer labs, this is a headache to most students too. If you want to regain such access to Facebook (and any other sites), just follow this simple step:
Open the command prompt (Start, Run, and type cmd) and punch this magical code
On the result, you will see some IP address say for example 126.96.36.199. Use it on the browsers address bar and whalla! You’re in. Change facebook.com to any site that was also blocked on your machine.