As expected, Facebook has unveiled App Center today, an "App Store" for iOS and Android that features both native and web apps that utilize Facebook integration in some way. The App Center features (only) 600 apps, and Facebook will offer personalized recommendations, presumably based on its social knowledge of each user. App Center will be featured prominently in the mobile on iOS and Android, Facebook's full dekstop site, and on Facebook's mobile site.
Facebook points out that it drives traffic to apps, pushing users to the iOS app store 83 million times in May. In that sense, it's not proper to think of the App Center as a direct competitor to traditional app stores. With only 600 apps, the App Center is hardly a comprehensive listing of apps — or even of apps that utilize Facebook's infrastructure in some way. Instead, Facebook is clearly hoping that it can use its strength to increase the popularity of apps that use Facebook in some way, which increases Facebook's popularity, and so on in a positive feedback loop. To help with that, the company says that it will only recommend "high quality" apps (read: apps that include Facebook in some way, even if only for authentication) and will also recommend apps that users' friends are using.
Facebook is adding a top-level "App Center" link to the lefthand sidebar of every desktop Facebook page. Once there, users can browse for apps in categories or search — and having found an app, they'll see your standard screenshots and descriptions. On mobile, you can directly launch the appropriate app store for installation. From the desktop, you can click a "Send to mobile" button to get a link for installation as well.
It should go live for US users at www.facebook.com/appcenter and within Facebook app updates soon.