Why do some Facebook games manage to retain and monetize users, and others fail completely? The answer to this is complex, but often boils down to the game’s design.
Some of the key components to a well-designed Facebook game are:

• Niche focus: Targeting a niche and being unique in that niche
• Fun and engagement: How engaging the core gameplay is to users
• Incentives to return: What incentives the game gives users to return (i.e. to check crops, when energy is recharged, etc.)
• Social interaction and identity: What kind of meaningful ways people can interact with friends (competition, sharing, cooperation, etc.) or other Facebook users and how can players establish themselves as unique among these other players.
• Advancement: A proper speed of advancement through the game
• Monetization incentives: How well the game incentivizes users to buy things to advance their gameplay.
• Content refreshes: How well the developer is able to keep fresh content to keep users engaged.