What is social media?

The best way to think about social media is word of mouth, or social interaction on different forms of digital media. Social media are blogs, forums, Facebook, Twitter, Google+, Youtube, Vimeo, RSS, Digg, Yelp, even World of Warcraft — online (and on mobile) is a form of social media.

Why does it matter? Social media now accounts for over 20% of time spent online.