I've seen the argument in the past that FB is a requirement for parents to communicate with their kids and their kids' schools as a justification for permitting it where it would be otherwise blocked.
Course I've also seen situations where the person(s) in charge of the filtering whitelist whatever sites they want or friends want, irrespective of policy. When I started my last job, one of my first tasks was to clean up the web filtering as it had been run by the helpdesk guys and they managed to misconfigure aspects of the appliance. Went in & noticed all their personal favorite sites that should have been blocked according to policy were whitelisted, unblocking them for all users. Edit: Really though, there's all sorts of possible reasons for inconsistency in blocking from the provider itself to the people running the filter & directives from management.