I would like to advertise this game more but I believe that I refer a friend system with very minor rewards would greatly increase recruitment.
Small things such as a 10% discount on the game 200 gems for the first time your friend buys gems In the store and/or 400 gems when your friend gets to 5000 achievement points.
May be an exclusive skin or teleport to a friend ( course you have to add teleporting to friend link to the gem stored to be fair to people played so far)
Sure people can come up with other rewards for recruitment.