This study aims to figure out the discourse functions of code switching which appears in the texts posted on SNS, especially Facebook. In order to satisfy this purpose, this study observed and analysed several individual timeline pages whose owners have a plural language background, but live in a monolingual society. The results are as follows. First, a lot of code switching examples appeared in the observed texts, even though the writer lived in a monolingual society. Second, the unmarked code of the individual writers was their mother language and it was chosen when the posting was about the writer's everyday life and appeared as a longer passage. Third, when the writers chose their marked code, they had at least four particular reasons that were related to discourse functions; including or excluding a particular reader, using more effective words, emphasizing or watering down the implications of the text, and changing the speech act or topic. Finally, these results show that the community on SNS is closer to a plural lingual society rather than a closed monolingual community.
On SNS, the users act as if they live in a multi lingual space and perform as plural linguals, although, in the reality, they live in a very monolingual society. And this is one of the important characteristics of a SNS discourse community.