EyeHawk, thank you so much sharing your stats.
The freemium model is the reigning king right now. Although i believe it doesn't work for all games, it seems to be the one model where most revenue is generated. I would agree that it would be best to add an IAP architecture to your game since your level hierarchic is set up for this type. The best practice is to get the game if front of as many users as possible and wait for a better return in the long run.
I will check your FB for updates.