Facebook Indie Games

1. Innovator phase -- non-social game play. The game needs to work even if other people aren't playing it at all. You probably won't make much money in this phase, but your players will be pretty forgiving and happy to give thoughtful, useful feedback. Even with very few users, you can work on retention and revenue-per-user before exposing the game to a bigger audience. (During this phase you might even limit invitations so that your players feel even more special, and let you test your game without spiraling costs.)
2. Early adopter phase -- socialize with other players. Give players ways to engage with other players, whether they are real life friends or not. Build a player community. Encourage players to recommend your game, grow by real word of mouth rather than underhand viral tactics. These still aren't really social games in that they don't rely on your social graph -- they simply use social networks as a convenient delivery mechanism for persistent games.
3. Mass growth phase -- bring in your social graph. This is where you want to take your game mainstream. Give players incentives to bring their friends into the game. Use viral tactics that border on spam to get people in. As players become less enthusiastic (which they inevitably will if your game becomes mainstream), you need to make it harder and harder for them not to spread the word.
4. Maturity phase -- play with friends. Farmville is now in this phase. The growth in player acquisition has peaked and they now need to keep their players interested for as long as possible. So they've introduced features where you can co-operate with your other Farmville-playing friends on big projects. While the "growth phase" co-operative features were built on bringing people into the game, "maturity phase" co-op features are built to make it harder to leave -- you'll be letting your friends down if you do.

Zynga uses a huge advertising budget to leap over stages 1 & 2, going straight to a stage where masses of players are joining and spamming their friends to get them to join too. Most indies can't do that.

Many indies might stay at Stage 2 indefinitely. Mousehunt is there. I keep playing it, and there is a thriving player community -- but none of my friends are part of it. Mousehunt offers some incentive to bring friends into the game -- but not much, and those features are quite hard to find. Certainly not in-your-face attempts at viral growth. Many indies might decide that Stage 3 involves entering a pact with the devil that they don't want to make. On the other hand, I think it does hold Mousehunt back a bit and they should try giving players more incentive to get their friends involved if they want to grow the game.