Typically I develop my websites on trunk, then merge changes to a testing branch where they are put on a 'beta' website, and then finally they are merged onto a live branch and put onto the live website.
With a Facebook application things are a bit tricky. As you can't view a Facebook application through a normal web browser (it has to go through the Facebook servers) you can't easily give each developer their own version of the website to work with and test.
I have not come across anything about the best way to develop and test a Facebook application while continuing to have a stable live website that users can use. My question is this, what is the best practice for organising the development and testing of a Facebook application?