Forget everything else for a moment and consider that your game is just a graph of users over time. There is more than one kind of viable graph, but knowing which kind you are aiming for is important. It should affect every strategic or marketing decision that you make. This week's news that Draw Something's user numbers have already dropped by 30% is significant, for example. It makes Zynga's purchase of it look like a lot of money spent based on misunderstanding that game's graph.
Do you know which kind of graph you are creating, and are you making the right choices to improve it? Are you targeting your monetization strategy in the right way, or are you basing it on a faulty understanding of its likely graph?