Update: Works on New 3DS only – Smash Bros. 3DS doesn’t support Miiverse on 3DS
Update: Miiverse can be used when playing Smash Bros. on a New 3DS unit. It won’t work on regular 3DS models due to insufficient memory.
At the moment, there’s no Miiverse support for Super Smash Bros. 3DS. There’s simply no way of posting to Nintendo’s social network currently. Whether or not this will change in the future remains to be seen!