Why choose a social website over a standard website?
Standard websites convey information in a single direction. The visitors to a standard website are strictly consumers. There is no interaction between the visitors themselves and the visitors are not able to make any contributions to the website. This means the site owner is 100% responsible for keeping their website's content fresh, and getting their visitors to come back for more. For example, if you're a blogger, this would mean you may need to come up with multiple blogs, and if you ran a large website with multiple categories that each target a different demographic, it could very quickly become a daunting task, even for a small team of writers and editors.
The advantages of a social website.
Social websites allow your website's visitors to become contributers, and are generally designed to encourage such contributions by providing features like commenting and voting. By encouraging visitors to create their own content we give them all a reason to keep coming back, and as a side-effect your content's life is extended beyond just a single viewing.
How does a social networking site extend the life of your content work? It's actually pretty simple: Normally, someone will hit an article's page once, read it, and then never come back to it. But if you allow commenting, people will keep coming back to the same article to see what others are saying about it or even to comment on other comments.