This study uses a conceptual model that integrates three research streams – knowledge creation and sharing, social capital, and online communities – to explore how social capital facilitates the creation and sharing of knowledge in online communities. It incorporates data from three well-known multinational firms, namely, Adobe, Dell, and Starbucks (software applications, computer and IT peripherals, and a coffee franchiser in a service industry, respectively). Our results show (1) why and how knowledge is created and shared, cultivated in online communities and then adopted by firms; (2) that social capital positively facilitates the creation and sharing of knowledge in online communities; and (3) that the process of creating and sharing knowledge nourishes the following outcomes which benefit the firm: it accelerates the development of new products, enhances relationships with partners, raises the level of environment and community involvement, creates customer acknowledgement, and intensifies product and service innovation.