Steve Streza (lead platform developer at Pocket) has seamlessly integrated App.net’s private messaging and file APIs into OS X’s Messages app:
The people you follow will be shown in your buddy list. Double clicking one will open a new chat window and start a new private messaging session. Sending and receiving messages happen very fast, usually within a second. You can drag files into the chat window, and they will upload to your account’s App.net file storage and be sent as part of a message.
The Messaging API is analogous to Twitter’s direct messages, though far more flexible (can include more than two participants, etc.)
I really hope that by now people have realized that ADN is not a Twitter clone but instead a series of APIs that can be used to build a wide variety of different applications. Say what you will about App.net but this is something that simply couldn’t be built on top of Twitter due to their API restrictions.