Felipe Ortega wrote a Ph.D. thesis devoted to this topic. "Wikipedia: A Quantiative Analysis", available at http://libresoft.es/Members/jfelipe/phd-thesis His conclusions were featured on a Washington Post article, "Volunteers Log Off as Wikipedia Ages", discussed here http://news.cnet.com/8301-1023_3-10403467-93.html and here http://news.slashdot.org/story/09/11/25/160236/Contributors-...