The service purposefully limited to naturally content-light context-light 180-character quips—whose intent is to broadcast and be broadcasted to unfiltered—achieves its destiny.
The result is natural and unavoidable. I'm so happy we're beginning to realize it.
Seriously though, this is one of those unmeasurables that's difficult to nail down. Sure you could blame it on changes to the UX, or on the user-base itself, or any number of other reasons. But in the end, you've gotta look at the nature of the form of communication itself and its parameters and assumptions, and make a conclusion at face value.