Realistically that's the growth curve experienced by even the most popular and addictive games (grow quickly to large # of users, slowly decline into mediocrity) -- the trick for Turf will be to figure out how to become more than just 1 game. There's no reason they can't launch new games within the game (just off the top of my head, think scavenger hunts or group battles), and get people engaging in all sorts of gameplay.