What particular factors might have begun to inhibit participation in 2006, when we know it began to stagnate?
I have done a new analysis of wikipedia's growth. It seems that the new-article rate it fits fairly well a two-phase model: exponential growth (double every 11 months) until 2005, exponential decay (1/2 every 5 years) since 2006:
New article rate N'(t) - linear scale
New article rate N'(t) - log scale
The dots are the data, the solid lines are the model. The fairly abrupt transition in 2006 rules out the "low hanging fruit" theory, and the steady decline after 2006 (instead of a sudden drop and gradual recovery) rules out "bad media image". The best explanation that I can think of for the shape of that graph is by assuming that
- over 90% of the new articles are created by regular editors (as opposed to newbies)
- a reader only feels the need to register after creating one article as a IP user.
- regular editors leave or become less active with a half-life of 5 years or so
- the rate at which new editors were recruited was growing exponentially until 2005
- that rate dropped to nearly zero in 2006
- the cause was some change in wikipedia (not an event in the outside world)
- regular editors were not affected by that change
Assumptions 4 and 5 seems necessary to explain the exponential growth of the regular editor corps until 2005, and the lack of growth after that. Assumptions 1 and 7 seem necessary to explain why the new article rate did not drop immediately when the recruitment rate fell to nearly zero. Assumption 3 then explains the decay since 2006. Assumption 6 seems the only way to explain the abruptness and persistence of the 2006 drop in recruitment. Finally assumption 2 provides a possible explanation for that drop: namely, the policy that prevents article creation by IP users, that closed the main and most natural path through which readers used to become regular editors.