Express yourself on a game-centric social network
Facebook and Twitter are great, but most of your friends probably don't give a shit that you just beat the seventh castle in New Super Mario Bros. U. That's where the Miiverse comes in.
It's a social network just for the Wii U, with a cross between Twitter and a message board focused on individual games. When you beat a tough level, a game might ask you if you want to write a message or draw a small picture to celebrate, then others can see it in-game or from the Miiverse application itself.