One thing that any social media web community needs is members. Members are the commodity of social media websites. The more members, the more value--not just to the site owners, but to the community itself. In fact there is a critical mass to a social media site. Without enough members the site has virtually no value. But as the community grows, its value increases exponentially. If a community only has a few dozen members there is not enough content to make the site compelling. Also, without enough people a community will not be self-correcting. James Surowieki does a fantastic job in his book, The Wisdom of Crowds, in describing how crowds (as long as they maintain decentralization, diversity, and independence) come up with better answers and more accurate analysis than even the most qualified experts would provide on their own. One reason is that a large, diverse group will tend to corroborate correct information while canceling out errors. It's why Wikipedia works.
Well populated social networks are have all the characteristics of a wise crowd. Del.icio.us pulls together the collaborative judgment of millions of people thus categorizing and ranking the sites among billions.
The benefits of a social media website is found in the connections that get drawn between those that participate. Connecting people, recommending resources, identifying expertise, and distributing content are all benefits to those who participate in and use these sites.