Social chatbots are increasingly designed to offer companionship to humans to improve their psychological well-being. Can they develop social relationships with the technology? How are people influenced by chatbots? Studies testing whether humans will adapt their communication when interacting with a chatbot have provided somewhat mixed results. Applying Communication Accommodation Theory (CAT), this study takes a longitudinal, quantitative approach to answer these questions. It employs natural language processing (NLP) techniques to compare the linguistic patterns in 29,885 conversations between human users and their social companion chatbot, Replika, scraped from Reddit. The results showed that people’s linguistic styles gradually converged with that of the chatbots. This convergence was positively related with the relationship development between the user and their Replika companion. These findings suggest that human behavior can be influenced by chabots through interaction and people can develop relationships with their technological companions.