Apparently people play FarmVille and continue to play it because of the social obligations that tie you to it. According to SUNY Buffalo instructor (and student) A. J. Patrick Liszkiewicz:
"One might speculate that people play Farmville precisely because they invest physical effort and in-game profit into each harvest. This seems plausible enough: people work over time to develop something, and take pride in the fruits of their labor. Farmville allows users to spend their in-game profits on decorations, animals, buildings, and even bigger plots of land. So users are rewarded for their work. Of course, people can sidestep the harvesting process entirely by spending real money to purchase in-game items."
You're warned that the link will take you to an article where the author doesn't understand why people play FarmVille and thinks it's both boring and addicting.