CityVille has grown from 0 to 90 Million Users in 30-days...insanity. The game is very well designed. From happy pixel citizens to cute tail-wagging dogs, CityVille keeps you emotionally glued to the screen in ways only a behavioral psychologist could understand.
Kevin Rose, Digg's founder has deconstructed the "social sharing" of CityVille, levels 1-3.
To grow their user base Zynga uses Facebook wall posts and target friend invitations through their "Goal/Reward/Share" system.
What's Goal/Reward/Share (GRS)?
Goal: Present the user with a specific goal. Example: build a house, or open a bakery.
Reward: After the user has completed the goal, present them with a reward, Example: new level achievement, more coins, or a gray bunny.
Share: This's the key to their growth, after the user receives the dopamine hit of their achievement, Immediately present them (on the same screen) with the option to tell the world about their new found status (ego stroke) and Share the wealth with their friends. Remember, this's always framed as a positive thing for both you AND your friends. For e.g., common share button copy reads: "share the news and coins", "share XP", "share coins now", and "send gifts to friends".
The flowchart below cataloged all actions that trigger a Facebook wall post or targeted invite: