When you write a Facebook app that hooks into the Facebook API and a user allows/authorizes your app to access friends and post on their wall etc, there is noth

When you write a Facebook app that hooks into the Facebook API and a user allows/authorizes your app to access friends and post on their wall etc, there is nothing stopping your application from storing all that "retrieved" information into a database of your own. You can do this because Facebook isn't hosting any of the files that make up your app. They all live on your own servers.