Today Facebook showed off their upcoming App Center platform that among other things will now allow apps that cost money to be sold. Other than being the first time developers can charge for their apps (besides micro-transactions) developers can now create an app detail page that will be listed in the store if it gets high enough ratings.
The store will also be available on mobile. It appears as though some mobile apps may work in iPhone/Android browsers while others will be linked to the appropriate app store for download. Recommendations are given through personal and social cues and not sorted by popularity. Some users suspect this is coming because Facebook will soon be traded on the stock market and wants to show how they have different ways to acquire revenue besides ads - which many are saying do not work. Read tons more about this upcoming Facebook feature on the official blog post.