From Lessig excellent book Remix about Wikipedia

Not all of the work within Wikipedia is writing original articles. Indeed, the vast majority of work is editing content— correcting spelling or formatting errors, rewriting submissions to conform to the NPOV norm, or simply “softening [a claim] to be more broadly acceptable.” According to one estimate, only 10 percent of all edits add substantive content. The rest is cleaning up those additions. And even here, more of the work is done by a relatively small number of users. According to Jimmy Wales, 50 percent of all edits are done by 0.7 percent of users— meaning just about 524 users within his sample. The most active 2 percent (1,400) of users have done 73.4 percent of all edits. Counting content, Aaron Swartz found that “the vast majority of major contributors are unregistered and that most have only made a handful of contributions to Wikipedia.”

Some of the references cited are Contribution Patterns Among Active Wikipedians by Seth Anthony and Who Writes Wikipedia? by Aaron Swartz.