Pokémon Shuffle has hit 3.5 million downloads, and in celebration a Celebi is being offered to players via a challenge. (Thanks, Serebii.net)
The Celebi challenge will run from today until May 18th. You will receive 1000 coins for completing the challenge the first time as well.
Pokémon Shuffle is one of two free-to-play Pokémon titles released in the last six months, the other being Pokémon Rumble World. How the latter game is doing is unclear at this point.