Waku Waku Blitz! was a freemium Facebook game with a gameplay similar to that a whack-a-mole. A match-3 combo mode was added to get higher scores, and a market granted the players access to new features and customization.
The economic model was very simple: the player gets coins each time he plays. The higher his level, the higher the salary. At first, the items are really affordable but around level 10, the price of the items asks the player to play more, to buy coins or to share items with friends. Little boosts on experience points or for time for instance, would act as a coin sink.