The genius in how Farmville has succeed in getting so many people addicted comes down to how it handles commitments on a player's time: every time you play Farmville and plant a crop, you're making a commitment to come back during a 12 hour window or so to harvest your crop, or else you forfeit your investment.
via Gamasutra: Mark Newheiser’s Blog – Farmville, Social Gaming, and Addiction.