Facebook (etc) is blocked where I work as "Personals and Dating"* - but like others have said - if you allow personal web browsing then why not facebook as well? A good solution would be the limit on time, for example, we allow 1 hours personal lunchtime browsing on entertainment, sports, etc, but still block facebook, a better solution would be to allow facebook as well at lunch times... if someone does have 1 hour for lunch and wants to spend it on the internet, why block facebook, but sports is okay for an hour? What difference does it make when people are on lunch?
* incorrectly in my mind - surely it should be categorised as "Social Networking"