Twitter and Facebook is integrated at a OS level with iOS. A few lines of code gets me access to the user's accounts (with their permission of course).
A bit more generic than that; there's no global setting where user says "this is my preferred App.Net app". When user hits Share in any app (many apps provide such a feature), a menu appears with all available apps user might want to share to. Any app can opt in to be in the menu, based on MIME type of what's being shared.
Apps can register to handle URIs, which can be local, or something like http://*.reddit.com/* so whenever a link to reddit is found in say an email or web page, you can choose how to handle it, either via the browser app, or a reddit client.
