I think this is just like standard forums, it all depends on the relevance of your promoting. Why not try and engage yourself in a way that positions you as an authority on the subject and build up a rapport with the other users first? You might even be able to get the page owners to link to your site or talk about it if you are helpful enough.
I don't think it's a bad etiquette. I do it sometimes, but only at my own personal facebook account where my friends can view it if they are interested. Posting on the wall of a popular page may not be unethical as well, just as long as you are promoting a good, informative, and educational site where the viewers can gain good things, not bad, especially for the very young ones. They are the most who are fond of facebook thingy. :-)