Plenty of websites do manage to get a lot of free (moderation) and content.
However, generally the trick is to already have the prestigious website that people WANT to contribute to and be associated with.
I know of plenty of people that put quite a lot of time in to running someone else's site for them because it's a subject they are passionate about and they believe the website is one of the best examples, or they are well integrated in to the society.
See plenty of people coming out of nowhere to ask others to contribute to their new site and it's incredibly rare they get anything beneficial.