This research investigates the formation of business customer brand engagement and strong customer-firm relationships in a social selling environment. The study was prompted by the growing prominence of business-to-business (B2B) social selling, in which goods and services are sold to B2B customers directly on social media. The stimulus-organism-response (S-O-R) paradigm is used to model the customer’s perception and response. Primary data was collected from a sample of customers (n = 378) that follow B2B social selling sites on Facebook for business services including training, consulting, legal services, digital marketing, and information and communication technologies (ICT). Analytical techniques included Q-sort methodology to check reliability and validity and evaluate non-response bias and structural equation modelling (SEM). The findings showed that information quality, virtual interactivity and rewards influence business customer brand engagement, which in turn had a positive influence on strength of the customer-firm relationship. A moderation analysis showed that although system quality did not have a significant effect on business customer brand engagement in the full sample, this relationship was significant for technology customers. The main contribution of the research is that it highlights how social selling contributes to customer-brand engagement and the customer-firm relationship, identifying some factors that had not been observed in the currently limited and fragmented body of research into B2B social selling. There are several opportunities for theoretical and empirical research to further expand the literature on social selling and B2B use of social media marketing, which is highlighted in the study.