In September of 2008, we blogged about a curious change in Wikipedia that we didn't know how to explain that we had known for a while, and the ASC group has been looking into understanding this change in the last 6-9 months or so. The change that we were curious about was that the growth rates of Wikipedia have slowed. We were not the only ones wondering about this change. The Economist (archived here), for example, wrote about it.
We are about to publish a paper in WikiSym 2009 on this topic, and I thought we should start to blog about what we found.
Monthly edits and identified revert activity
The conventional wisdom about many Web-related growth processes is that they're fundamentally exponential in nature. That is, if you want some fixed amount of time, the content size and number of participants will double. Indeed, prior research on Wikipedia has characterized the growth in content and editors as being fundamentally exponential in nature. Some have claimed that Wikipedia article growth is exponential because there is an exponential growth in the number of editors contributing to Wikipedia . Current research show that Wikipedia growth rate has slowed, and has in fact plateaued (See figure at right). Since about March of 2007, the growth pattern is clearly not exponential. What has changed, and how should we modify our thinking about how Wikipedia works? Prior research had assumed Wikipedia works on a "edit begets edit" model (That is, a preferential attachment model where the more an article gets edits, the more likely it would receive more edits, and thus resulting in exponential growth .) Such a model does not preclude some ultimate limitation to growth, although at the time it was presented  there was an apparent trend of unconstrained article growth.
Monthly active editor - number of users who have edited at least once in that month
The number of active editors show exactly the same pattern. The 2nd figure on the right shows how since its peak in March 2007 (820,532), the number of monthly active editors in Wikipedia has been fluctuating between 650,000 and 810,000. This finding suggests that the conclusion in  may not be valid anymore. We have a different process going on in Wikipedia now.
Article growth per month in Wikipedia. Smoothed curves are growth rate predicted by logistic growth bounded at a maximum of 3, 3.5, and 4 million articles.
Some Wikipedians have modeled the recent data, and believe that a logistic model is a much better way to think about content growth. Figure here shows that article growth reached a peak in 2007-2008 and has been on the decline since then. This result is consistent with a growth processes that hits a constraint – for instance, due to resource limitations in systems. For example, microbes grown in culture will eventually stop duplicating when nutrients run out. Rather than exponential growth, such systems display logistic growth.
We will continue to blog about what we believe might be happening in the next few weeks, as we find time to summarize the results.
 Almeida, R.B.m, Mozafari, B., and Cho, J., On the evolution of Wikipedia. ICWSM 2007, Boulder, Co., 2007.
 Spinellis, D., and Panagiotis, L. The collaborative organizations of knowledge. Communications of the ACM, 51(8), 68-73, 2008.