Tripadvisor is a good example of an organisation relying on the free input of its users. The question is what do they get in return. This blogpost gives the answer:

Maybe a week later, I received an email from Trip Advisor saying that someone found the review helpful.
Very nice touch. Without that email, I might not have gone back to the site or posted another review. Now I am starting to see some signs of encouragement.

There are more reasons given, together withe examples of the emails and badges that Tripadvisor uses.