This master’s thesis aims to analyze Thai–English code switching behavior in CMC social networks, hence filling an important knowledge-gap in the field. Data – online interactions by nineteen educated Thai bloggers – is drawn from Facebook. This thesis describes the general patterns of Thai–English code switching in online communities of practice, by means of a socio-pragmatic approach. Whilst Facebook represents a limited arena of language use, it can nevertheless be argued that it nicely shows the discursive practices of young Thai urban dwellers. Therefore, studying Thai Facebook data allows us to analyze code switching in a context where it becomes an expression of ‘trendiness’. This discursive function of code-switching is different from what has been so far reported on code switching in Thai, and so studying the Facebook arena provides an important contribution to our understanding of this pivotal discursive phenomenon.