Facebook's App Center will feature the option 'Send to Mobile', which will allow users to purchase apps on their desktop, or tablet, and will have a notification delivered to their phones. This is something already found in Google and Apple's app stores, but instead uses Facebook's notification system.
This option is said to be [hopefully] ready in time for the App Center's launch later this year. The way it works is you'll choose an app through the App Center, and you'll have the option to directly launch the app if it's not already installed, or go to the iOS App Store, or Google Play to download it if you don't.
The App Center will be the first storefront to sell apps for both platforms together. The social networking site won't be directly selling, or making money off of iOS or Android apps. Facebook want a tighter grip on the mobile market, and hope to build its App Center into the main repository for socially-oriented apps across all of the major mobile platforms.