Social Media implies 'media'. Media means - 'Push'. Traditionally, media is a way to push your message out through various channels, be it television, newspapers, direct mail, radio and now online - via Tweeter, Facebook and LinkedIn.
Social Media, however, is not media. It is an Action. It is a conversation with two more people. It is not just a website. Facebook without a conversation is nothing. So, Social Media broadly defines how you converse online with others - be it customers, colleagues, or influencers.
Taking the example of Twitter as a platform for social media, if you use it just to send out ads and other company updates, it is just media - not social media. Otherwise, you are just crashing a party and handing out flyers.
If you are writing a blog, however thought provoking, if it does not have the ability to invite comments, it is not 'social'.