Image Credit: App.net
The App.net Messaging API which is also called the "private messaging API" can be used by app developers for a variety of use cases such as mobile messaging, instant messaging and collaboration tools.
According to the API documentation, the App.net Messaging API is built around two central objects; messages which are like Posts, and channels which are like streams of Posts. Developers that would like their apps able to create public messages that can be seen in App.net streams, a Post should be created with the Stream API.
Developers interested in using the App.net Messaging API can sign up for a developer account, which at the time of this writing, costs $100.00 per year.