Entities such as Wikipedia or Linux have always been a bit of a mystery to economists as to why people with great knowledge donate their time to write articles or software. Some are motivated by pure altruism, others by professional credentialing that accompanies being a leader on software like Linux. [See Jochai Benkler on Wikipedia, Linux and the gift economy in "Coase's Penguin, or Linux and the Nature of the Firm." or crowdsourcing]
In any event, the number of volunteer editors on Wikipedia fell last year by 49,000 (a jump of 10-fold over the prior year’s loss of 4,900 editors). There is active disagreement whether this has resulted from their being less new ground on Wikipedia as more and more things have been covered or whether editors are put off by increased bureaucracy Wikipedia imposed in an effort to increase the accuracy of Wikipedia articles and decrease the mischief. Moreover, Wikipedia has become less friendly to new contributions: “In 2008, Wikipedia’s editors deleted one in four contributions from infrequent contributors, up sharply from one in 10 in 2005, according to data compiled by social-computing researcher Ed Chi of Xerox’s Palo Alto Research Center.”
Despite this, Wikipedia’s popularity continues to grow: “Indeed, Wikipedia remains enormously popular among users, with the number of Web visitors growing 20% in the 12 months ending in September, according to comScore Media Metrix.”
One interesting snippet from the article is that 87% of the volunteer writers on Wikipedia are men.
The article does point out that Wikipedia founder Jimmie Wales is more interested in web traffic to Wikipedia and accuracy of the articles than in the volume of volunteerism on the site.
See: Julia Angwin and Geoffrey A. Fowler, “Volunteers Log Off As Wikipedia Ages“, Wall Street Journal, 11/23/09.