it may be helpful to think about virtual goods not just in merchandising terms, but also in terms of premium content. People have always paid for content like movies or concerts because of the emotional experience they get from watching or listening, even though they don’t end up with anything physical to take home at the end of the day. In this sense, social games are like a much more interactive form of entertainment media that also happens to allow for a wide variety of inexpensive a-la-carte premium content purchases that enhance the experience.
Inside Facebook ponders the relation between virtual goods and the service (business) model. I agree that virtual goods can provide an emotional focus point, yet they do not have narrative aspects in a similar fashion, and therefore I do not see that the movie/concert comparison is totally accurate. Especially in their functional and decorative aspects virtual goods are, in the end, objects that afford collecting and use, which makes them different from a scheduled entertainment consumption experience such as movie-going.