Facebook's just announced the App Center, an app store that it's says "gives developers an additional way to grow their apps and creates opportunities for more types of apps to be successful." The center will be rolled out on Facebook itself, as well as its iOS and Android apps. Listed apps will be downloaded from Apple's App Store and Google Play, not Facebook itself. In effect, this is all about promotion. You can begin the process of listing your app now, if you're ready, or just absorb the details. They're on Facebook's announcement, here.