From my personal experience, internal Wikis are 90+% written by about 5% of the workforce. The vast majority of internal Wiki users are "read-only". That is fine. Really: Read that twice. It is OK that the vast majority of docs are written by the few who are highly motivated to do it. When I write for an internal Wiki, I am trying to reduce the number of future questions that people will ask me. If they do come with a question, my first answer is always: Did you search our Wiki? It only takes once or twice for them to change.