Mental Floss clues us into an interesting article on MediaCommons about why we play Farmville — basically, because we've been trained to not be able to ignore social obligations.
The secret to Farmville's popularity is neither gameplay nor aesthetics. Farmville is popular because in entangles users in a web of social obligations. When users log into Facebook, they are reminded that their neighbors have sent them gifts, posted bonuses on their walls, and helped with each others' farms. In turn, they are obligated to return the courtesies. As the French sociologist Marcel Mauss tells us, gifts are never free: they bind the giver and receiver in a loop of reciprocity. It is rude to refuse a gift, and ruder still to not return the kindness. We play Farmville, then, because we are trying to be good to one another. We play Farmville because we are polite, cultivated people.
I don't play Farmville, but I do keep my Facebook page pretty app-free because I fear getting entangled in such obligations.