There's probably some linguistic confusion surrounding the word 'media'.

WhatsApp is without a doubt a medium for social activity, in the same way that telephone lines and the postal service are.

However 'media' has kind of stopped being the plural for 'medium' and is now a term on its own. This means you can interpret also social media to mean the social exchange of information media, though perhaps it is more accurate to consider them an extension of the other broadcast media as social media seem to be increasingly in the grips of media magnates who fill it with advertising.