The best way to define social media is to break it down. Media is an instrument on communication, like a newspaper or a radio, so social media would be a social instrument of communication.
Think of regular media as a one-way street where you can read a newspaper or listen to a report on television, but you have very limited ability to give your thoughts on the matter.
But social news is not the same thing as social media anymore than a banana is the same thing as fruit. A banana is a type of fruit, but fruit can also be grapes, strawberries, or lemons. And while social news is social media, social networking and wikis are also social media.