We examine how entrepreneurial firms leverage online social networking communities to establish legitimacy. We propose that one measure of a firm’s legitimacy is the size of its online community, and capture an intermediate measure of legitimacy this is associated with credibility. We develop a theoretical model of how entrepreneurial firms grow their online social communities through the use of symbolic actions, and hypothesize about the specific ways that the use of four categories of symbolic actions can facilitate subsequent growth in a firm’s online social community. We empirically test these hypotheses and find that actions that convey product and industry knowledge, and firm achievements are significantly associated with online community growth. This may indicate a form of preferential attachment wherein people want to associate online with firms that they perceive to be knowledgeable and have noteworthy achievements. Our study’s non-significant results for the influence of actions conveying professional organizing and capability of key team members on legitimacy contrasts with the findings from past studies, and thus highlights the importance of differences in the audiences evaluating legitimacy. Contrary to initial expectations, we find that seek opinions has a negative impact on online social community growth.