I won't knock FB for the advertising upside. But those 100k won't really be users of the service, which is what you really want for something so community-centric.
How many people signup for sites like these, and return rarely or never use any of the registration-enabled features? Are those "real" users as opposed to FacebookApp-enabled users?
I add an app in my Facebook. If it's popular enough, every few days I get a bump in my newsfeed from a friend who's actually using the app (albeit within FB). Splash some ads on the landing pages or figure out a freemium model, and all of a sudden it starts looking like a real business (embedded via iframe/fbml!).
