The most important benefit of monetization models based on virtual items is that there is no maximum amount that players can spend on each game.

In the traditional retail model, the maximum amount earned per gamer by a publisher is mostly limited to the purchase price .In free-to-play games, the game experience can be monetized virtually forever, as long as the game engages its user base. Obviously, the biggest challenge for this type of game is convincing enough people to become paying customers.
Currently, the large majority of gamers do not pay any money to play Facebook or free-to-play games. Most titles monetize only 5-10% of their active player base.
Facebook games
Facebook games (also referred to as social games) have some unique characteristics that set it apart from other online games.
• Social games are layered on top of social networks. Theyutilize the underlying social network platform and need toconform to its guidelines, which usually include privacy,spamming, and monetization constraints.
• Social games are asynchronous multiplayer games. Gameplayis designed as asynchronous (instead of synchronous, asin traditional videogames). This setup eliminates the requirementfor players to be online at the same time as their opponentsand allows social games to be played without hugecommitments of time.
• Social games are extremely viral and can scale up theiruser base in a short period of time. Zynga’s FarmVille gained10 million daily active users in its first six weeks in the market;CityVille reached 20 million users in 11 days1, making it thefastest-growing game in history.
• Social games are mostly casual in nature and require littleup-front development costs. However, since social gamesare ongoing services, expertise in providing Internet servicesand customer care is essential to a company’s success. It isnot uncommon for operational and service costs to accountfor 70-80% of a game’s total cost. Finding cost-effective infrastructuresto accommodate huge customer bases of millionsof players is critical.
• Social games provide impressive amounts of user data.Game developers are able to fine-tune their games afterlaunch, based on users’ behavior. They can detect lapsesin user interest and take measures to revive interest in thegame (such as launching new virtual items, special offers,etc.). These steps are nearly impossible (or prohibitively expensive)to do with traditional boxed games.
Parks Assoc Trends in Digital Gaming White Paper (PDF 2 Mb)
(Source: Parks Associates 2/2012)