Exponential growth, followed by a plateaue phase.

Thanks for this analysis! It's mirrored some of the work that PARC and others have done, some of which is on this wiki. Philippe is currently undertaking the Herculean task of merging all this work into one place, and I hope he'll include this as well.

Regarding your question about plateauing project participation: This is the great unanswered question. The best hypothesis I've heard so far: the worldwide economic downturn.