defamatory statements on social media

I have a guy who posted hotel reviews on trip advisor. The reviews about his experience and ratings about hotels. It is voluntary contribution. Some points are added with each review. These are posted along with the responses if given by the hotel managements. I examined a review and corresponding statement by hotel manager. My impression was that the facts stated in review are not precise, incomplete and some case lies, wrong and some times defamatory. Are not the social sites responsible for publishing or encouraging reckless reviews that is published and offending rights of parties people brought in question. I found one such review and hotel manager stating facts that caused me damages. How can I find the reviewer and ask him to pay for his wrongful review.