First off, do you have a policy, backed at a high level, that forbids Facebook? If not, then you may be treading on the toes of your boss, or their boss, who actually want to use Facebook. Many companies are happy to accept reasonable levels of social network use at work anyway, and view an all out block as counter productive. I'll assume there is a banning policy in place though.
If you worry about circumvention then you'll need to start investigating blocking Tor or Freegate, and then the next thing and the thing after that - but I'd advise monitoring your network for evidence that these, or other circumvention techniques are in use, and a friendly word in the ear of the guilty party on first instance, and their manager on the second. Usually just letting someone know you can tell what they are up to is enough to get it under control.
Anyone with the smarts to use Tor or other "clever" ways around should also be smart enough to use Facebook on their mobile phone, which you can't block.