The use of big data by large food retailers is increasing their bargaining power against the agri-food cooperative sector. The aim of this study was to determine the social media behaviour of food retailers in Spain and the UK, and to identify significant changes pre- and post-COVID-19 pandemic. The study analysed Twitter data collected from 16 food retailers; a total of 102,200 valid tweets were extracted from their official Twitter accounts. A term frequency analysis and a social network analysis of food retailers’ Twitter behaviour were carried out. The results obtained show differences for both UK and Spanish retailers before and during the COVID-19 pandemic. For agri-food cooperatives with little bargaining power in the supply chain of fresh produce, data analysis is a key factor in improving their competitive positioning. These findings should be of value to data scientists as well as managers responsible for forming strategies in agri-food firms that have large food retailers as clients. Finally, the study also confirms that, for agri-food cooperatives, analysing tweet content is a cheap and useful tool for understanding customer behaviour.