The Social Games work with a “effort-reward” system, generating what is called “compulsion loop,” or “engagement loop”: when a player gets a small reward for each action performed, he will be motivated to act continuously, in a “gratification loop”, or a “involvement loop”.
People act continuously with the prospect of receiving rewards: money and love are a concrete example, but even a smile  can lead a person to live a better day. Similarly, achievements and virtual goods force players to do significant actions within a game: they allow players to communicate and spur each other continuosly. Virtual goods provide a way to check and compare players’ actions and progress in-game.
The free-to-play business models induce players to buy and sell virtual goods, in a cyclic compulsive efforts system, capable of generating rewards continuosly, just a vicious cycle that leads users to search for achievements for everything (such as Jesse Schell says). Rewarding the user with a continuous reinforcement, weighing and evaluating every little action, is a tactic that affects player’s behaviour: driven by their own greed, players are forced to invest real money in order to obtain immediate satisfaction.
For game designers is, therefore, important to set up an optimized environment, accordingly to an efficient obtain-and-share system, inside the gameplay experience.
At the same time, virtual goods have to be funcionally meanful, and able to provide special attributes for players who require it as a reward, enhancing the gaming experience. Another method, is to to increase the virtual goods customization rate, in order to trigger the exchange. This result is obtainable also with the “famine” condition: limited resources increase the demand for virtual goods.
Bringing players to compete and cooperate (considering their individualistic behaviour), deliberately creating limited resources, the designers can establish and increase the game perceived value.