As far as blogger churn, I have not seen in depth data on this but just a few things from technorati

http://ideas.blogs.nytimes.com/2008/11/11/silence-of-the-blogs/

and

http://www.businessweek.com/the_thread/blogspotting/archives/2007/04/blogging_growth.html

(1) Beyond search visibility, it may simply be a fundamental attention limit. Wikipedia received increasing site rank (according to Alexa.com) up until about summer of 2006 to about the 8th most-trafficked website in the world. It then became very difficult to get any more traffic. This fundamental attention limit may have had an effect on overall growth. There is simply a fixed amount of user attention to Wikipedia-oriented topics (ie., things that are more or less encyclopedic).

(2) Our later blog posts will examine these barriers (usability, community guidelines, rule limits), and how they affect the Wikipedia editor population. In a nutshell, newbies experience disproportionate resistance compared to more experienced editors, and this resistance has been increasing over time.

On churn, we have some results showing how the patterns have been changing over time. It would be very interesting to compare it against blogging data. (Do you have pointers to blogging measurements that show saturation in the blogging space?)

It’s unclear whether Wikipedia is entering stagnation or if it needs to switch to a maintenance mode. We will continue to monitor and contribute scientific understanding where we can…

1) Traffic: It’s important to note that Wikipedia’s traffic is often tied to search visibility for topics. Google is responsible for over 45% of referring traffic to Wikipedia, with Yahoo! coming in next at 9%, so search visibility is a huge traffic driver (see referral analytics). Yet, growth in search has not been exponential as well so that is perhaps correlating to what we are seeing here. This TechCrunch post charts search activity and while Google is growing — not exponentially of course because it is a pretty mature product — other search engines remain flat. (It is also interesting that Knol has not managed to take any real traffic away from Wikipedia.)

2) Contributors: Wikipedia’s contributor model is based on committed and experienced volunteers. There is a limit to this number of users because there does seem to be a learning curve with syntax and guidelines that are a barriers to continuous growth beyond the core group of contributors. I would also be curious to see if an organization like Mozilla is facing a similar issue in its contributor growth model. There could also be a certain “churn” factor for contributors as they can simply get burned out in doing the work; this is something that can be observed in bloggers as many do quit because of the commitment needed.

Wikipedia does seem to be entering a stagnation phase and that is never a good sign.