I was extremely surprised to hear on the radio yesterday that the Shebab (the group responsible for the mass killing and ostage situation in the kenyan mall) does its public communications through... Twitter, of all things.
A silicon valley webapp used by somalian terrorists. The modern, "connected" world is weird sometimes.
To answer your question, and although I don't use it myself, some of my friends tell me they use it a bit like IRC: when you want to discuss of a particular topic, instead of joining a particular channel you ask your question with a specific hashtag (hey, IRC channel names are hashtags after all!) and it starts a discussion. It's like a more fluid IRC where chatrooms are created, deleted, merge and fork all the time. It sound good in principle, but I like the open and distributed IRC more.