Social media, Web 2.0 based social activity and powerful marketing tool, brought about a new paradigm for the 21st century culture and society. In line with the trend, the fashion industry is vigorously undertaking marketing activities on social media, seeking to build and maintain relationships with their customers through the Internet and mobile devices. The one-way provision of information by firms to customers has evolved to a symmetrical form of customers, whereby the customers also supply information to the firms share information with other customers. Through social media services, customers not only share their thoughts and experiences but also exchange tangible products and publicize the products, thereby directly participating in production and marketing activities of brands. Such participation of consumers offers inspirations for differentiated service on the part of the brands, for which proactive and long-term participatory activities of customers are required. Customer participation is of primary importance in current service marketing industry. Until now, related literature on customer participation has mostly centered on offline service situations. However, the advent of social media has simplified the process of participation for customers, and invoking voluntary customer participation has become the greatest challenge facing service industries, and, at the same time, the path to success. Those of us who live in the age of information technology are offered the opportunity to engage in two-way communication with other customers, the brand, and the media. Thus, customer participation is just as, if not, more important in an online environment. More and more customers are becoming directly involved in brand activities, which affect the brands as well as the customers themselves. Nonetheless, extant customer participation research focuses on developing constructs that are largely based on offline situations. Although such constructs correspond in part to the online setting, they fall short of meeting the unique characteristics of customer participation in social media. Therefore, in order to establish constructs for customer participation in social media, revision and supplementation of the offline-based constructs were found to be necessary. Based on interaction theory, this study explored the notion of customer participation in fashion brands’ social networking site (SNS) service by categorizing it into three types: customer-customer, customer-brand, and customer-media participation. In addition, it examined the external factors that influence customer social participation and how active social participation of customers in SNS affects trust and customer equity. The association between measurement variables according to the brand type was also closely examined. This study underwent two pre-tests to revise and supplement the survey items on customer social participation. A preliminary investigation was conducted on 516 respondents, and a total of 582 respondents participated in the main investigation. The respondents who were invited to participate lived in Korea, were in their 20s or 30s, and had previous experience in using fashion brands’ SNS service, SPSS 18.0 was used to conduct frequency analysis, exploratory factor analysis, reliability analysis, and t-test. Based on the results, AMOS 18.0 was used to undergo confirmatory factor analysis and structural equation modeling. The results of the study are summarized as follows. Based on the brand type (SPA and luxury) and the rate of sales, Uniqlo, a global SPA brand that boasts the largest number of SNS fans, and Louis Vuitton, a global luxury brand, were chosen for the study. The respondents consisted of consumers in their 20s and the 30s who, within the last six months, had previous experience in using the SNS service of the brands. The sample consisted of 40.7% male and 59.6% female, and 72.5% were in their 20s and 27.5% in 30s. A large percentage of the respondents were students (64.9%). The large proportion of university students reflects the trend of most active users of SNS being university students. Second, SNS service shopping context found positive association with customer-customer participation and customer-brand participation, but customer-media participation was insignificant. The results show that as the SNS is characterized by its linkage to different websites and platforms, customers are exposed to constant opportunities for participation. On the other hand, customer-media participation was found to be insignificant, which can be interpreted as indiscriminate constant supply of information that seeks to spur impulsive buying leading to antagonism of the customers. Such negative situations can be avoided by departing from indiscriminate provision of information to offering information that is customized to the needs and demands of each customer and invoking the customer to participate in information delivery. Third, SNS participation motivation and customer social participation were all positively associated. In particular, customer-media participation demonstrated the strongest influence, followed by customer-customer and customer-brand participation. As such, active participation of the customer with the brand is the most effective way to create collaborative, synergy effect or co-creation. Fourth, all routes from SNS user orientation to customer social participation were significant. SNS users in general possessed high levels of self-efficacy and tended to feel elevated as well as find altruistic pleasure in providing information that would help others. Such users participate through numerous ways such as customer-brand and customer-media participation but were the most active in customer-customer participation. Fifth, it was found that customer-customer and customer-brand participation builds trust of the customers toward the SNS service and the information or the information provider. However, customer-media participation did not indicate the same results. In order to build trust through participation, brands need to provide an inviting environment for customers to post comments and share information. Furthermore, marketing strategies, such as offering events and coupons to stimulate E-WOM activities of customers as well as creating a space for open communication are vital to building trust. Sixth, customer-media participation was positively associated with customer equity. Unlike customer-customer and customer-brand participation which requires constant physical effort of the customer, customer-media participation demands considerably less time and effort apart from the initial stages during which customization process is undergone to receive tailored information with just one click. Therefore, establishing convenient and practical system that targets the Y-generation in their 20s and 30s is emphasized in order to maintain a long-term relationship. Seventh, trust is positively associated with customer equity. Building trust is based on interactivity among people. This study found that customer-customer participation and customer-brand participation influence customer equity through trust, while customer-media participation leads to customer equity without being mediated by trust. Therefore, all three types of participation are equally important to customer equity. Eighth, brand type was a meaningful moderator to the relationship between SNS service shopping context, SNS service user orientation, participation motivation, customer social participation, trust, and customer equity. To summarize, the type of participation to encourage is based on the brand type, and as customers may have different expectations according to the brand, it is important to understand their orientations and provide customized information or an open space for communication. In addition, participation behavior has a strong potential to influence customer equity, but depending on the type of participation, trust may not necessarily positively influence customer equity. Therefore, it is recommended that brands offer credible and sincere information that customers can trust and follow. The significance of this study is that it is one of the first studies to examine customer participation in a social media environment. At a point in time where customers’ value creation is attracting increased attention, this study empirically revealed that customer-media relationship, which enables customer-customer and customer-brand participation on SNS, is also within the bounds of participation. Furthermore, this study identified the importance of relational marketing approach to brand and customer through customer social participation. By applying the concept of customer equity, which is in its fledgling stage in the area of consumer fashion marketing, it offers a basis for future research on long-term relationship building with firms The limitations of the study and suggestions for future research are discussed below. The sample of the study consisted of Koreans between the ages of 20 to 30. The limitation in the sample is inadequate to generalize the findings, and therefore, a comparative study with broader sample across culture and age is necessary. Moreover, this study observed customer participation on SNS environment. However, as there was a lack of preceding literature in this area, the measurement scales were revised and supplemented to fit the purpose of this study. The measurement instrument, therefore, needs to be further applied with different antecedents and consequences in subsequent research. Finally, due to the lack of previous literature on customer participation in SNS environment, the theoretical basis of the relationship between the variables lacked solidity. This study adapted a part of a theoretical framework to fit the context of the study to come up with the hypothesized relationship between the variables. Therefore, it is suggested that future research works toward developing and strengthening the theoretical framework of customer social participation.