In the second decade of the 21st century, social media changed the nature of communication and cooperation between participants of the culture services market. They became, among other things, an important marketing instrument in the area of contact with the customers of the cultural offer. However, despite their growing importance in various areas of activity of organisations in the cultural sector, the issue of building the cultural institution's brand equity by social media users is relatively seldom raised. Research on the impact of online consumer activity on brand equity is at an early stage of development. Therefore, this article is an attempt to fill the research gap in this area. The article presents the results of a survey conducted in 2018 on a group of 1021 consumers of cultural services, who at the same time regularly used social media. The statistical analysis carried out and the research results obtained prove that the 3C sustainable system developed by the authors, concerning the activity of consumers of cultural services in social media, stimulates the consumer-based brand equity (CBBE). Statistically significant relations have been observed in particular for CBBE components related to the awareness of a cultural institution's brand and for the relationship related to the perception of its quality. The study opens a review of literature on social media and consumer-based brand equity (CBBE). Next, based on the COBRA model (consumer's online brand-related activities), a proposal of the 3C sustainable system, concerning the activity of consumers of cultural services in social media, has been presented. The further part of the article presents research hypotheses, a conceptual model, research methodology as well as results and conclusions. The last part of the article discusses the results obtained and indicates the existing management implications.