This paper examines the language choice and ethnic identity construction online of four young Chinese Indonesians from West Kalimantan, Indonesia. The study draws on a combination of Social Identity model of De-individuation Effects (SIDE) theory and audience design theory to formulate a hypothesis about participants' linguistic behaviour within different social circumstances online. Broadly, the hypothesis stated that participants would adjust their language choice and self-presentation to suit different online circumstances. Online social variables such as levels of anonymity, audiences, group identity and personal identity were predicted to impact on participants' linguistic behaviour. The hypothesis was evaluated through analysis of online interviews with participants and observations of participants' interactions on two social networking sites, "Kaskus"® and "Facebook"®. The paper shows that participants draw on their multilingual linguistic repertoires to construct their ethnic identity in different ways according to the social variables they encounter on each site. The results are generally consistent with the hypothesis of the study; however SIDE theory and audience design could not definitively explain the code-switching practices of participants in interactions with others of the same ethnicity on "Facebook"®.