For all of its glory, Twitter is apparently not as sticky as many social media buffs would like it to be. A recent Harvard Business School study reported that 10 percent of the service's users account for more than 90 percent of tweets. (I wrote about Twitter's lack of loyalty back in April.)
However, I don't think it really matters. As with any service or piece of software, a rising tide lifts all boats, so a core user base can propel a service for quite a while. Somewhere down the line however, Twitter as a company will need to put programs and efforts into place to encourage people to actually use the service if it ever plans to monetize it.
The fact that 10 percent of users are driving 90 percent of the content is not dramatically different than what you see with sites like Wikipedia, or with personal blogs, which have an even lower rate of consistent publishing. According to a 2008 study by Technorati, 95 percent of the blogs they track hadn't been updated in at least four months.
Orphaned tweets, like orphaned blogs, are just as much part of the social fabric as anything else. The fact is that people abandon stuff all the time--TV shows, books, whatever. We shouldn't be remotely shocked that someone bails out of blogging or something else that could be considered work.
If you are not motivated in some way to make the effort to blog or use Twitter you will sooner or later realize you could be doing something else. That's the great thing about choice--but it's also the risk for companies like Twitter that haven't yet figured out how to make money.
There is another parallel to be drawn with open-source development. A core team of developers are typically paid by a company and write 90 percent of the code, while the last 10 percent or so may done by the community.
Any community or user-generated content falls into the traps--a mass of people need to be motivated to do things for some kind of gain. Without a core group of contributors there will be no momentum at all.
Follow me on Twitter @daveofdoom.