Basically what Rey C and Petra said. There should be no expectation of privacy on facebook whatsover. Unfortunately a lot of models are naive about how the internet works. There are girls that make the decision to pose nude and then are horrified to find out that their nude pics are circulating throughout the internet. (Personally I think this could be minimized if paysites and webmasters would not post the nude pics on promo sites or on the free sections of thier paysites, therefore you might get more models willing to pose nude, but that is another discussion).
If you enjoy the model's pics on facebook, if there are a lot of them, then I would save the pics and not tell anybody about the facebook page, if anything so that she will continue to post the pics.