With the evolution of social media, social search has been taking a shape of its own. There are a number of social search products that have emerged and become very powerful in a very short period of time. Search Engine Watch recently published a two part series written by Chris Sherman which first describes social search and its history and then introduces all the various types of social search products currently available. The second article breaks the social search products into six categories:
I would highly recommend checking this article out for more detailed analysis of each of the six categories.