Neilsen reports that 60% of new Twitter users do not return after the first month. I think the issue is content. There isn't enough great content. I'm on Twitter and I follow a handful of people, mostly in the marketing space. I send tweets every so often, maybe 5 times a week, but I'll admit it isn't anything groundbreaking.
Twitter is a fantastic communication tool, but it doesn't scale well. The more people you follow, then harder it is to keep up. Yet, media companies are evaluating all these new methods of communication the same way: by scale. They care about eyeballs and viewers and numbers. These metrics are old and crusty. Just like the music industry is undergoing a major shift from physical distribution, communication companies must adapt to the concept of "small is the new big".