Readwriteweb just published this info: 22.5% users post 90% of tweets. It just confirms that, like most consumer product, the 80/20 rule applies for Twitter as well.
Now, what's really interesting is the growth of people follow more than 100 friends. It went from 7% on 2009 to 21% in 2010. That's a huge achievement. Those folks are much less likely to churn.
It's easy to think that those are probably also the same who contribute tweets. But let's be crazy. What if not?
What if Twitter is becoming a reading tool where you pick your topic(s) and just watch? This is just a personal opinion and an intuition, but I feel more and more people are just reading Twitter vs contributing to it.
We may start to see two very distinct categories of twitter users: the publishers, and the readers, while in the early days, everyone was contributing.
The interesting part for Twitter is to create an experience for those readers, making it easy for them to access relevant information and consume it.