In this study, the use of a popular mobile social networking platform (WeChat) is investigated for learning Chinese as a foreign language (CFL) in a beginners' course at an Australian university. Driven by practical considerations and major issues with previous literature, student engagement and authentic language used in a WeChat class group are investigated. Data was collected through weekly conversation episodes over a semester and from a student survey. Conversation episodes were analysed both quantitatively (by frequencies) and qualitatively (using a discourse approach). The study found that student engagement increased in frequency and complexity towards the end of the semester, and students used Chinese for various authentic purposes in the semi-formal context of class group chats. A microanalysis of transcripts illustrated how the use of Chinese in multilingual and multimodal practices achieved authentic social functions. However, individual differences were observed, and English was frequently used in chats. Recommendations are provided for scaffolding and for utilising different semiotic resources in dynamic group interactions.