It is very difficult to make a game buy-once, play-anywhere because the various platforms (Google Play, Facebook, iTunes, etc) don't talk to each other. Not surprisingly, each platform wants to receive the profit from a sale and has no incentive to enable you to play a version of the game that you purchased elsewhere. This is why nearly 100% of games from all developers where you pay upfront on (for example) iTunes must be purchased separately on Google Play. It is also worth noting that it is actually quite a lot of effort to put our games on every major platform. Each platform requires a significant amount of work and results in significant ongoing support costs. So even if the platforms talked to each other, it would be financially very difficult for us to enable players to freely download a game on every platform after paying just once, on one platform.