One the biggest drivers of user contribution to Wikipedia is clear mission statement they have. I have actually not read it and I am writing this blog in a airplane, but as a user and contributor to Wikipedia I have somehow got the message.
"Create a comprehensive and accurate free encyclopedia in 10 years."
Why Wikipedia has 10K+ active contributors?
Passionate contributors: They feel that they are participating in something that outlast their lives. When you add something there you are reasonable assured that it will outlive you. It is a way making history.
Large pool of authors: Everyone is a an expert. Everyone has something they are good at and want to pass that along. As individuals they may not be so smart but as a collective they are a genius.
Low barrier to entry to the author pool: The wiki collects and aggregates micro contribution for every users: In a successful and well maintained wiki you, as a contributor add what you can. You can forget about spelling grammar or style, if you have content and what to add it you can just dump it in. There will be others who clean it up and format it. If you are not an expert in a subject then you can contribute by simply formatting and editing. Just consuming the content is also a form of contribution. Page view statistics help authors focus their effort on a subset of content.