Online brand communities can be valuable to firms, but how do firms cultivate such communities? We find that engagement, that is, "likes" in response to firm posts, in the online brand community is associated with subsequent growth in the community. We theorize that when individuals engage with firms’ posts, the social media platform broadcasts such interactions to others, who are not necessarily part of the firm’s online brand community. Such social diffusion of information about the interaction and the related firm content provides individuals with new information about the firm, based on which they may decide to join the brand community of the firm. We find that firm posts that convey firm credibility through product and industry knowledge, convey organizational achievements through information about firm milestones, partnerships, or awards, seek opinions, and convey promotions or offers, are associated with engagement. Such posts have a significantly greater effect on engagement for early stage brand communities, that is, for those in their first year and a half, than for later stage brand communities.