From FredCavazza.net
The Social Media landscape has been split up by Fred Cavazza into four main subcategories depending on the usages (expressing, sharing, networking,playing) and structured around social platforms which try to cover each user’s needs.
'SOCIAL' NETWORK ANALYSIS OF THE MEDIA ECOLOGY 
There are a number of different conceptualizations of the social media domain. This is a nice way to divide up the space.