The definition of media is:
Media: the means of communication, as radio and television, newspapers, and magazines, that reach or influence people widely
I placed emphasis on widely. It's about as true that Facebook or Twitter or any other Social Network is Social Media as is the telephone. A telephone is a tool. Facebook and Twitter are tools. They provide a gateway through a medium.
Medium: an intervening agency, means, or instrument by which something is conveyed or accomplished: Words are a medium of expression.
We don't all sit around and watch Facebook on our computers, we interact with it and utilize it to communicate with others. As a medium, it's important for Marketers to recognize it as such… this means they can't simply post something out there and expect something to happen, they need to participate to make it happen.