AppNetKit is for Objective-C developers interested in working with the App.net API. The library is asynchronous and uses blocks to notify you of completions. As of late August 2012 this library interfaces with all portions of the app including: token, users, and posts.
The Facebook Graph API is great and in one year has thoroughly upgraded Facebook's approach to its platform. The Graph API is much simpler than previous iterations of Facebook's APIs and Connection methods. Of course, with the simplicity of the Graph API, it's still easy to get overwhelmed by just how much information you can easily (with explicit and secure user permission, mind you) get access to and use in your apps. Facebook employee Simon Cross put together a handy Facebook Graph API Explorer to help you navigate the Graph API response content.
Twitter announced the Periscope Producer API, which will give third-party apps the ability to authenticate Periscope accounts and let people push live video from just about anywhere. The idea is to make it easier to share video from just about any device and reduce pain points for end users.