Paper by Oded Nov, published on Communications of the ACM (November 2007)

A random sample of 370 Wikipedians were emailed a request to participate in a Web-based survey.
A total of 151 valid responses were received (40.8% response rate), of which 140 (92.7%) were from males (first “gosh”!).
The respondents’ mean age was 30.9, and on average they have been contributing content to Wikipedia 2.3 years.
The average level of contribution was 8.27 hours per week.

The Wikipedians were asked to state how strongly they agree or disagree on a scale of 1 to 7 with items.
Items were related to 8 different types of motivations: Protective, Values, Career, Social, Understanding, Enhancement (typical measures about volunteering motivations) and Fun, Ideology (added by authors since relevant for Wikipedia).

Overall, the top motivations were found to be Fun and Ideology. Agreement with Fun was in average 6.10 (in the range 1 to 7!). Ideology was 5.59. The other motivations were inferior to 4.

Each of the six motivations positively correlated with contribution level.

The Ideology case is particularly interesting (…): while people state that ideology is high on their list of reasons to contribute, being more ideologically motivated does not translate into increased contribution.

It would make sense for organizers of user-generated content outlets to focus marketing, recruitment, and retention efforts by highlighting the fun aspects of contributing.
Credit for image: nojhan released under Creative Commons