Adiran Hon talked about many other mechanics that social games use to keep plays playing a game. Each mechanic a way of persuading a player to keep playing through compulsion. Simple in game mechanics have the ability to persuade a player to keep playing to the point of addiction. These mechanics usually use phycology to help them work.
Avoidance:
This is using punishment as a tool in which to bring a player back into the game to achieve better. When a player loses in a simple way then they would be disappointed but if it was simple then theres a high chance there determination would increase to not fail or lose again if its easy not to. Avoidance in Farmville is known as the withering crops as if you sign in to late you may loose value or not getting any money for the crops you made. This means players lost in money and would hopefully get them to sign in at the right time. The negative effect also might keep them checking the game as they don't want to be punished again.
Variable Ratio Reinforcement (rewards):
Its the same mechanic slot machines use to keep gamblers going as every so often a payout will rewarded. This rule can apply for games with plays being rewarded points, coins and objects ever so often. It is created to change behaviour as a player who would usually sign in once a week to play may play more if there was a high chance of 'winning' or getting rewards. Its an insensitive to keep playing for more rewards and in most games you are working towards a goal to get a reward less work for better outcome is a good reason to keep playing.
There are many games that use this method from social to AAA console games. Farmville uses daily rewards based similar around lotto as variable coin rewards are given for signing on every 24 hours. These rewards are variable as they are random to how much or how little you get. This persaused the user to keep going on for the best chance at getting a high payout and even if they don't its still free rewards.
Games like halo use credits as rewards for completing a game. these are random at the end of a game but what they have also done is added a large bonus for those who regally complete games. Randomly you may get a huge payout for not quitting and playing a lot thus keeping players in the game. Its creating the players to think they are getting something for nothing but what they are doing is getting addicted and keeping loyal to the game.
http://blogs.telegraph.co.uk/technology/adrianhon/100006049/a-new-breed-of-computer-games-is-creating-compulsive-behaviour-someone-is-making-a-lot-of-money-out-of-this/
