Facebook integration into iOS was kinda-expected. However, the complexity involved of Facebook sharing is much greater than that of Twitter. Facebook is a multi-faceted monster. There are lot more elements to the Facebook experience, than a 140-character-limited text box.
For example, you can upload a picture to your Photos or post a link to a photo on the News Feed. In addition, Facebook has the added layer of metadata, such as tagging and album-sorting and privacy settings.
Also, there are just a lot more types of content that Facebook can handle. Will you be able to share a Calendar appointment to Facebook? Will you be able to sync Contacts with your Facebook contacts?
My guess, for the time being, it will be limited to just News Feed posts and OS-level API support, so Facebook-login to apps is streamlined and managed from the global device Settings. This also means there will be a prominent option to disable the feature, akin to the Twitter implementation.