A new system update for Nintendo 3DS released yesterday finally brings the Miiverse and Network ID  to the Nintendo 3DS. Players can download the update for free, below is more info :

The update will allow users to register a Nintendo Network ID to their 
      Nintendo 3DS system. If they also have a Wii 
      U system, they can use the same Nintendo Network ID to combine their 
      Nintendo eShop balances and Wish Lists. A Nintendo Network ID is 
      required to download free content, like game demos, from the Nintendo 
      eShop on Nintendo 3DS.

      In addition, Miiverse support has been added, allowing Nintendo 3DS 
      users to connect with players around the world to share experiences and 
      game tips. While the full Miiverse experience on Nintendo 3DS requires a 
      broadband Internet connection, there is also an offline mode for players 
      without access to Wi-Fi. Users can queue up to three posts while in this 
      mode, and those posts can be manually posted to Miiverse once a Wi-Fi 
      connection has been established. As with Miiverse on Wii U, users can 
      include select screen shot content from either the top or bottom screen 
      of the Nintendo 3DS system.

      The update also lets Nintendo 3DS owners transfer all of their content 
      to different Nintendo 3DS systems an unlimited number of times, once 
      every seven days. Before the update, users could only perform a 
      system-to-system transfer a total of five times.