A new paper released by the Social Computing Lab of HP Laboratories discusses the core social network beneath the web of friends and followers a person may have across many social media sites.
By studying over 300,000 Twitter accounts they developed some parameters of what defines a friend as opposed to a follower or followee: in this case to become a friend two posts had to be directed to another user.
Not one to give away the ending I highly recommend you give the paper a read. For you non-academics out there the writing is digestible albeit dense and worth the time invested.
Social networks that matter: Twitter under the microscope
Bernardo A. Huberman1 , Daniel M. Romero1,2 and Fang Wu1