How do you feel when you have a million of people around the world working as a group working together to make the site information accurate and up to date?
It is what drives site lik <a href="http://www.wikipedia.org/">Wikipedia</a> and <a href="http://www.youtube.com/">YouTube</a> so successful. The content was checked frequently in Wikipedia by many to make sure each entry is truthful and unbias. YouTube was able to protect itself away from offensive material by users themselves to make sure the video were not offensive. The information were so much that it is hard to keep track of by just a few people, or even a hundred of them. It is better to maintain by the people who view or create the material. Similiar to open source software, the more eyeballs on the source code, the easier to point out security risk.
Of course, it comes a cost.
Wikipedia has recently suffer from inaccurate reports. Changes in those report were made so frequent that Wikipedia has to limit who can change the content.
YouTube has million of copyrighted material like TV shows hosted on the site. When the users _think_ it is OK to share those video, then it is up to the copyright agency to control it. Again, the copyright agency has just so many employee to control what material should available on YouTube.
After all, why does this community oriented site become a trend? I personally think that was related to more people receive education. Back then, only a priviledge fews receive education. There are only a small group of people in their little circle exchanging information or able to teach. Corrupted few would even prevent teaching others in order to keep their "superior status". However, as education become more accessible to more people, many learn that sharing information was the key to improve themselves and the society. Internet becomes a tunnel to access to obtain or provide infomation as a website becomes a marketplace to anyone who wants to know about anything.
It is a win-win situation when information are available to view by many and update by many.