One things stands out in my conversations with people about social media these days: a lot of folks aren't using Twitter as much as they did say six months ago.
I hear a lot of reasons:
"It is too noisy"
"I hear from the same people over and over again"
"I just don't feel like saying anything"
The net of the feedback is that, as a mature platform, Twitter makes more sense for the "established" than for the "long tail". If you have built a large audience, it's great. But if you haven't, you'll spend a lot of time and possibly money making it happen. And at the end of the day, you become just another loudspeaker alongside every other member of the "Twitterati".
Regular people don't just share thoughts in a 140 character canvas. They share their personalities, their pictures, their opinions backed by links on the Internet, etc. For most people, investing time in Facebook is a much richer experience with the added benefit of being a place where at least your friends will pay attention to you.
Twitter has to evolve into something more than a loudspeaker to survive. If it doesn't, Twitter will be dominated by the same voices we've always heard (for the most part)... all with the same loudspeaker and the need to be heard. The fact is that everyone needs a voice and nobody likes being drowned out by the loud and obnoxious who feel the need their opinions and agenda with the world. If Twitter just becomes another way to listen and not participate, you can count a lot of people out.