Twitter has decided to kill several editions of their popular TweetDeck app to focus on developing "modern, web based" versions, with the iPhone, Android and AIR editions biting the dust. The references to these apps have already been removed from the TweetDeck website, and the apps will be removed from their respective stores in early May, before they cease to work entirely.
Twitter and the TweetDeck team believe the official Twitter apps on Android and iOS will suffice for users, with more users switching to these apps in favor of TweetDeck as new features have been added (such as photo filters). TweetDeck are now focusing their efforts on the web and Chrome versions of the app - editions which will get features first - while the Mac and PC apps will also continue to be developed.
TweetDeck was acquired in May 2011 by Twitter for around $40 million, and at one point commanded a significant amount of market share for posting updates to the service. Since the acquisition, numerous features have been removed, and with todays update it looks like the client will return to being Twitter-only, as Facebook integration will be discontinued.