The short answer seems to be "to have fun."
Oded Nov at the Polytechnic University, New York recently surveyed Wikipedians about their values and the extent of the time they spent contributing to find out what motivates them. Nov asked contributors to Wikipedia about how much they endorsed various value statements about the degree to which contributing to Wikipedia was a labor of ideology, helpfulness, overcoming negative personal feelings, career enhancing, a chance to learn, or in response to others encouragement and/or just fun.
Wikipedians reported that the main reason they contributed was that it was fun and when Nov correlated this value with the time spent contributing there was a strong positive correlation. Interestingly, the other strongest correlation with the level of contribution was overcoming efforts to overcome personal problems.
Nov doesn't ask any deeper questions, but we are left with a puzzling finding. Is Wikipedia an effective way for people struggling with personal problems to have fun? Are there two different groups of people-- those motivated to have fun and another that is motivated to overcome difficulties. And what does this suggest about encouraging participation in the Web 2.0 world? Should fun be the primary goal?
Note: This report was published in the Communications of the ACM, 2007, 50 (11), 60-64.