A Social Network (ex: Facebook) is a collection of applications that relies on a common messaging, user management, and authentication infrastructure.  Typically, this infrastructure is characterized by the data structure used to represent users and their connections to one another.  This data structure is called the social graph.  

Social Media is a more generic term used to describe content, applications, or advertisements that leverage social platforms (ex: Facebook Connect).  Social platforms are services that help connect said content, applications and advertisements to users leveraging one, more Social Networks. 

With those definitions in mind, the concept of Social Networks is anachronistic.  Social Networks are dead.  Social Platforms are the future as traditional media is quickly becoming replaced with Social Media.