Facebook and Twitter are no different than Emails and Messengers. These days they are forms of communication and should be open to all employees for use. If one use the net for non-business matters he can do it, through Skype, Windows live or any other network... it's true that it's harder to avoid it with Facebook, but with time it will just be another form of communication.
This strip sounds like where I work. We got an email requiring us to check the organization's facbook and twitter accounts at least daily. Too bad they are "social networking" sites and are blocked. PHB is everywhere!