資訊科技與網際網路的日新月異，許多企業的商業模式成功關鍵取決於產品網路外部性(Network externality)的使用人數連結價值，例如：Facebook社群網站、亞太電信網內互打免費方案和Apple Inc.推出互補性產品iTunes Store刺激iPod的銷售業績等。這些商業模式的競爭法則是當企業或某一商品或服務擁有較多的安裝基礎(Installed base)時，則更易於在人際網絡中發揮網路外部性，啟動正回饋效果進而再度擴大使用者規模。本研究將探討在網路外部性影響下創新擴散的情形，並利用模型進行銷售預測分析。

傳統擴散模型(Bass, 1969；Kalish, 1985；and other)將新產品被採用之過程隨著不同產品生命週期階段描述出來，且可透過產品過去之銷售資料預測未來銷售發展，但在過去許多擴散模型的延伸研究中，所專注的都是消費性耐久財，普遍忽略了網路外部性效果對新產品銷售之影響，因此本研究以Bass (1969)的Bass模型為基礎，將現今網路外部性現象考量至模型中，發展出一個易於使用與預測之工具。以社群網站Facebook使用者人數與互補性產品如iPod及其線上音樂商店iTunes Store之銷售資料，分別作為直接與間接網路外部性模型之實證研究對象。

本研究發現模型中加入網路外部性因素的考量後，修正模型的模型配適度與解釋能力皆比Bass模型更準確，且分析產品關鍵多數(Critical mass)後，透過修正模型的參數估計值可以了解不同銷售時期下影響產品擴散速度的因素。另一方面，研究發現網路外部性強度會影響達到關鍵多數所需要的時間，因此廠商可依據修正模型參數估計的結果，作為協助企業行銷策略擬定和商業模式建立的參考依據。

For some product categories, such as fax, telephone and social network services, the utility of a product depends on the number of consumers who have adopted the product. For some others, such as DVD player, digital TV, computer and smart phone, it depends on the availability of complementary products. The former effect is referred to as direct network externalities and the latter is as indirect network externalities in the literature. In the market with network externalities, managers need to evaluate when achieving the critical mass, which is the minimal number of adopters of an interactive innovation for the further rate of adoption to be self-sustaining. The rate of adoption is slowly diffused in the early stage of diffusion, and that the rate of adoption increases fast after the critical mass is reached. Therefore, managers change marketing strategies over time in order to sale product efficiently.

Although diffusion modeling has been researched extensively for the past 40 years, it doesn’t consider the diffusion processes of the product with network externality. This study develops two models that consider the direct network externality and indirect network externality separately. The diffusion model for products with direct network externality was applied to forecast active users of Facebook and the diffusion model for products with indirect network externality was applied to forecast sales of iPod and iTunes Store. When compared to Bass model, both of them showed better performances in forecasting long-term sales. Besides, this study focuses on network externality and critical mass. Diffusion modeling, the study field in marketing that seeks to understand the spread of innovations throughout their life cycle, has adapted to describe and model these influences.

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