QUOTE
That can happen only because people see the worst content and the worst conduct being rewarded.
So they increasingly imitate the worst.
Since in a hobby the most tangible reward is the fun that comes with it, people will drift away from what they perceive as annoying or boring. On Wikipedia, someone may start making a page and putting it on the site. Then as time passes he will notice that things change, and generally the pages don't improve and get worse, and at that point where's the fun in making pages and maintaining them in a "cooperational project"? Therefore, if that someone decides to continue by his free choice, another way to have fun can derive from "grinding", repeating the same action over and over to attain more "power" (or cookies, brownies, bronze stars, ribbons, wikitutnum books, wikipetan dolls, shiny medal of the supreme master editor...). Even more fun in being an abusive character to others, spamming death threats, faking his credentials, moving pages to random titles...