Communicating with Twitter used to be possible via multiple channels: web, mobile web, SMS and IM. There are other communities like Twitter, but they are mostly web-based. Twitter was never strong on features or stability, but it ruled on accessibility and convenience.
IM support was disabled during the summer and Twitter always said they were working on fixing it.
Then Twitter shut down SMS updates and its usefulness went down dramatically.
Today I noticed the message about Twitter working on IM updates had disappeared and in Twitter's settings page it no longer sports any IM settings. Apparently the IM feature has been discontinued "indefinitely".
This means Twitter these days has no features a lot of other web communities would not have. In fact it has a lot less. The only thing Twitter still has are the users. It will take some time for the users to migrate elsewhere and a lot of users probably will stay. Especially the U.S. users never got used to using Twitter over SMS. But in the U.S. text messages are expensive and phone data plans are cheap.