I am an economist working on a research project analyzing contribution behavior on Wikipedia, and am interested in computing the fraction of individuals who use the site and make contributions.  I have found the data on daily contributions, and am now looking for data on the number of individuals who use Wikipedia. I believe I have found the data I am looking for here: 

Unfortunately, the data contained in that file only covers the period from August 2002 through October 2004. Does a similar database exist for later time periods?  Any information you can provide would be greatly appreciated.  Thank you for you time and I look forward to hearing from you.

Sincerely,

Nathan Marwell