they can do it by using software like websense, OpenDNS, Content filtering via router etc...
When I was in school I had the same problem. I used torbrowser and I was able to visit site's that were blocked. If you have websense blocking you then this might now work. I doesnt hurt to give a try.
google search : torbrowser bundle.
Maybe the technician had a good reason to block facebook at your job, for example avoiding you receiving a memo for doing non-work related tasks at your workplace.
Be carefull, it's not your personal computer at home.
Technicians block sites using a firewall or content filtering software, or can do it from the router.
Some dsl modems have content filtering as well, so its probably done through that module.
There are many out there, and some free ones based on linux as well.
The blocking can come about due to management decision, to make employees waste less time at work, of course.
It can also be the decision of the System Administrator, to save on Internet bandwidth that the company pays a high price for.
If you are using Facebook for personal reasons, why not use it at your home?
Of course, if you want to be unproductive at work, you could use proxy softwares, and sites like hidemyass.