Miitomo isn't released yet but you can find more info here https://miitomo.com/en/ and the OS info comes from FAQ what you can find here https://miitomo.com/en/faq/
Miitomo will be released sometime in mid March on the Play Store and App Store but untill then you won't be able too find Miitomo on them.
Also, go to https://miitomo.com and make sure to pre-register for the Miitomo app. By registering you will get bonus My Nintendo Platinum Points when My Nintendo and Miitomo launch in March.
The URL for My Nintendo is https://my.nintendo.com/ but the website is not available yet.
For more information in detail about Nintendo Accounts go to: http://en-americas-support.nintendo.com/app/answers/detail/a_id/15997
Miitomo
Miitomo is a social networking app where you can answer questions about yourself, and share them with a friend or worldwide through Answer Central. These messages are sent theough your Mii that can be transfered from your Nintendo Account or be created from scratch.
You can also dress up your main Mii and any sidekick Miis you make and then take fabulously customizable pictures known as Miifotos.
There's other stuff to do as well, such as frustratingly try to pachiko your Mii onto platforms to get costumes only available there or you can listen to questions from people across the world using the Answer Central.
For more info, see this website: https://miitomo.com/en/
Thank's for getting my hopes up. But I wouldn't be surprised if something like that happens. We never receive pokemon distribution and currently we don't even have Nintendo distribution of their games in Brazil (which means all the nintendo games arrive in stores months after their world release), the news of their reject to certain parts of the world don't sound that shocking to me to be honest.
edit: Also for their first cellphone app "Miitomo" not a single South American country is available to play, which alerted me for Pokemon GO might having the same issue (https://miitomo.com/en/countries/).
I'm guessing it's yet to be updated with an expiration date. The icon on the main page is also different, it doesn't have the "Nintendo 3DS" badge as the other themes and rewards.
If this is the 4th item described here, then it'll expire at July 13th https://miitomo.com/en/collaboration/160608_zelda/
The time for Pre-registration on https://miitomo.com/en/ ends on 3/31 at 1:00AM PT.. Whether or not this is any indication to the release time of Miitomo on the IOS & Android stores for the rest of the world is uncertain but it gives you a general idea.
All I know is that if you have the Japanese version of Miitomo, the update that launched earlier today v1.1.0, you can now change your country of residence and link your NA Nintendo account to the app. Before this was impossible because only "Japan" was listed in the countries section & as such it wouldn't let you link it together..
I linked my NA Nintendo account to the JP app, got the bonuses for pre-registration from the site linked above.. So I'm guessing they're aiming for a 1-app release, if this is the case I wonder how they'll handle the "Launch Period" bonuses that are ending in JP today & how they'll move forward with the "Launch Period" bonuses for the rest of the world.. Is the rest of the world being completely screwed in the sense that these events won't be coming to global making the Mario Hat & limited time miidrop pachinko rewards unobtainable?
I doubt this is the case, but it would be horrible if it were the case & most of the world gets screwed because Nintendo didn't launch worldwide at the start..
Apparently this was also addressed in the update: "Smaller app size. Data saving has been improved, resulting in a smaller, sleeker app."
source: https://miitomo.com/en/ver2/
edit: I haven't paid much attention, so I can't tell you how much space it took before and after the update.
Keep you hopes up. https://miitomo.com/en/countries/ states explicitly:
>◆ Additional countries may be added.
Edit: besides I think most, if not all, of the countries with their own eShop will be added also the countries that participated in Club Nintendo. Don't think Nintendo will only make this exclusive to the current group of countries, if only for the reason that it is easier to manage one system/network across the world.