Social media has become a value-adding marketing channel. With the significance of social media for online small businesses and the lack of understanding of this area, this study attempts to investigate the impact of social media marketing on the sales performance of such businesses. Drawing on the theories of media richness and uses and gratifications, we propose a research model in which the impact of social media marketing message strategies (i.e., message content and format) on customer engagement, brand awareness, and sales performance of online small business is investigated. Customer engagement and brand awareness are expected to mediate the relations between message strategies and online small business sales performance. To validate our model, we will collect data from a Twitter-like microblogging platform Sina Weibo and e-commerce platform Taobao in China. This study is expected to contribute to research and practice on social media marketing and online small businesses.