I have been using Russell Tarr's Fakebook for several years with my students. It allows the kids to create a Facebook like page where they can have friends, put up posts and make comments. It is not real in that the students are not sharing with others, but rather creating their own world. They also are given their own unique url and allowed to set a password of their own. BUT they do not have to sign up, or give a e-mail or anything else that will identify themselves. It is a great way to have students create a conversation between historical figures or even current politicians to show that they have learned the material successfully and can apply it.
Since Russell has not yet created a new tutorial to match his newly improved Fakebook site, I have my own above.