i recently finished the book "grouped: how small groups of friends are the key to influence on the social web".http://www.amazon.com/Grouped-groups-friends-influence-social/dp/0321804112
i encourage anyone getting started in (on-line) social networks to begin with this book. it's a neat, well-written and easy-to-read survey of the state-of-the art in the field, authored by a person who's apparently very familiar with the area. read it!
having said that, i also encourage anyone wanting to dig a little deeper to actually check out the sources used in the book; i guess there's more than one controversy lurking in what is here presented as facts. for instance, the notion of one being influenced by the actions of people up to three degrees away in your network, as introduced by christakis and fowler (referenced multiple times in "grouped"), has been contested (see, e.g., http://www.degruyter.com/view/j/spp.2011.2.1/spp.2011.2.1.1024/spp.2011.2.1.1024.xml