How would Facebook know who likes porn (except assuming everybody likes porn, obviously)? Those who say so on their Facebook profile are presumably not very concerned about that information leaking out?
Most are logged into FB when they browse their porn, so FB definitely knows what porn and how often the user browses it. Whether or not that info would end up in the graph, I don't know. I don't THINK it would? Not sure.
In any case, FB definitely HAS the information. Just a matter of whether they would let anyone else have it. I could see it being a valuable "background check" type service.
Actually, now that I think about it... FB tracks your browsing whether you are logged in or not. So they would probably have A LOT of information on people who like porn.