Strange juxtaposition of 2 different things imo. Facebooks paranoia or prudery has very little to do with the applicability or UK porno law apart from the face they were probably constructed by those of a similar attitude to "father knows best".
Rather than outrage bear in mind that the key part of the UK law is - interpretation. The reason this is left some what open to judges is so they can use their judgment. It may involve some idiot plod erring on the side of caution occassionaly but that's how our justice system works over here.