As explained by Matt Prior, a producer for FIFA 13, there’s an ever-present “Friends” tab on the Wii U’s GamePad. You simply tap this to open a list of your friends, from which you can either invite someone to a game, or send them a message. As far as the messaging is concerned, it’s:
…much better than the 360 Live virtual keyboard where you’re moving it up and down. …we really want to connect friends and allow people to game with friends as easily as possible.
http://wiiublog.com/matt-prior-talks-about-the-friends-list-in-fifa-13-on-the-wii-u/