What’s good about microblogging services like Twitter, and Identi.ca, is the simplicity and that all posts are available to everyone. This makes it possible to use these services in a number of different ways. On e.g facebook your updates are only seen by your friends, on twitter they are seen by everyone. This means you can listen in on things without having to be accepted as a friend, and in real-time even.
Myself I mostly use twitter and identi.ca to follow conversations and comments about usability and interaction which really helps me with my education.
Others use the services to promote their sites, share music, or keep track of their friends. Some use microblogging to maintain their schedule, or help them with physical exercise. There are so many possible uses and they multiply when you start to use external services, like twistory and the likes, and clients made for microblogging.
Twitter is very flexible, simple, adaptable and has asynchronous relationships, which is probably what sets it apart from other types of networks. Twitter doesn’t really support groups or community features so here myspace or facebook are much better for keeping in touch with friends, but if you mostly have friends on your twitter then it works for that as well, almost like IM.