From the Nintendo press release:

Don’t miss out on being a part of a Nintendo first! On March 31, Miitomo, the company’s first-ever smart device application, is launching in the United States and several other countries. Miitomo is a free-to-start social experience that lets users spark one-of-a-kind conversations with friends in a whole new way using Mii characters. The app recently launched in Japan and was downloaded more than 1 million times in its first three days of availability, while achieving the #1 most downloaded free app status in both the App Store and Google Play storefronts.

And:

After downloading the application for free on the App Store for iPhone, iPod Touch or iPad, along with Android devices, users create a Mii character and customize facial features like eyes and hair, as well as voice and personality. Users can also use the camera on their smart devices to snap a photo of themselves, which can be used to create a Mii, or they can simply import their Mii from their Wii U or Nintendo 3DS systems using a QR Code.

Once their Mii is ready, users can add friends who already have the app in person, or by linking the app with their existing Facebook or Twitter accounts. After that step is complete, it’s time to answer some questions! Miitomo creates conversations and turns discussions into a form of play by prompting users to answer all kinds of fun questions. How would you define your fashion style? What’s the weirdest thing you’ve ever eaten? If you were given ten grand to spend in one day, what would you do? The answers get shared among friends, sparking interesting conversations – part of the fun is discovering unexpected facts about your friends. When users hear how their friends responded to questions, they can give answers a “heart” or a written or picture comment.

To me, this is a foundational app to build a social network for future Nintendo app releases, moving the Mii characters into the smartphone universe.