I've been very tempted to take the Facebook App first approach on a number of projects. Like you say, it focuses you on what the core value of your application

I've been very tempted to take the Facebook App first approach on a number of projects. Like you say, it focuses you on what the core value of your application is without being distracted by the social networking fluff around the outside. If what you create proves to be really compelling, you should be able to convert a good portion of your Facebook users over to a stand alone version of your app at a later date. Obviously this only really works if enough people in your target market fit into the typical Facebook demographic.