For me, an addiction is a more complex behavior with more triggers. A compulsion is just to those discreet activity (washing hands, flipping switches, etc.). But "porn" can be many different things to different people and accessed in different ways. People who give up smoking, for example, have to pay attention to when and where and with whom they normally indulged. A compulsion doesn't care about who, when, or where; that stays the same each time, is more single-minded in its pursuit.