I honestly wasn't sure it would happen but App.net is actually starting to get interesting. Not so much for the core Twitter-like experience, though I've found myself using it more lately, but for what developers are building on the foundation of its APIs. Today the developers of Riposte, my favorite ADN client for iOS, have announced a new group messaging app called Whisper and it looks fantastic. If it's approved in time it looks like it could very well corner the market on social planning and communication this year at WWDC.
If you don't have an App.net account yet you can sign up for a free account (and get some extra storage space) using this invite URL:
Once you have an account, downloading the ADN Passport app will give you one-click access to many of the iOS apps.