This one is up to you. However, people tend to talk about Pinterest. It isn't nearly as big as Facebook, Twitter, or Instagram, but it is prominent and on the rise. You may have already noticed the "Pin It" share button appear on articles or seen the Pinterest icon on YouTube videos (it's right next to the Tumblr icon.) Vine is another one that has exploded lately. If your interviewer says these are too mainstream/not up-and-coming enough, try bringing up more budding platforms like Impossible, and We Heart It.