The in-game messaging system for
Pokémon Shuffle announced that over 1 million copies of the game have been downloaded since its release on February 18th, 2015. Players that Check In between March 2nd, 2015 to March 30th, 2015 will receive a free
Complexity -1 (パズルポケモン-1) item as thanks for downloading the game.
Source:
http://www.pokemon.co.jp/ex/poketoru/news/150302_01.html