Several papers address the issue of "wiki" communities evolution and viability. Thus, "Viable wikis: Struggle for life in the wikisphere" (2007) is a position paper on what may influence the (non-)viability of wikis as collaborative platforms, including a demographic study on about 7,000 such communities.
"Measuring Wiki Viability" (2008) (with Dario Taraborelli & Nigel Gilbert) then explores the demographics of hundreds of wikis over a period of several months, with respect to various demographic and organizational variables.
It notably shows that a stronger growth in terms of registered users is paradoxically associated with wikis open to anonymous contributors (who are not required to register).
"Viabilité des wikis" (2008) (again with Dario Taraborelli & Nigel Gilbert) summarizes these results towards a more sociological audience.