This study investigates the patterns and reasons behind code switching (CS) in Facebook conversations among Thai students who are learning English as a language (EFL). The research was conducted with 40 final year students at Nakhon Sawan Rajabhat University in Thailand all of whom were enrolled in an English for Government and Business Purposes class. The study used a mixed method research design combining text analysis and semi structured interviews. The quantitative analysis revealed that the common CS patterns were switching from Thai to English (TH+E) and switching from English to Thai (E+TH) highlighting the prominence of English as a shared language. Qualitative insights uncovered the use of Thai particles to convey politeness and respect emphasized how CS is used to express group identity and revealed its use for location specific context and conveying complex ideas. This research offers an understanding of CS dynamics in communication shedding light on how language, culture and identity intersect in online interactions, among Thai EFL students.