This study investigated the use of intrasentential code switching on Facebook. The corpus included 1000 screenshots of Facebook comments collected from 10 Saudi female Facebook friends who were bilingual in Arabic and English. The data were examined through statistical and content analysis. The results showed that intrasentential code switching occurs frequently in informal Facebook interactions. Further, the occurrence of code-switching was analyzed in relation to 10 topics of Facebook interactions, including gossip, humor, technology, compliments and thanking, achievement, movies and songs, family and intimacy, makeup, travelling, and religion. Among these 10 topics, gossip and humor elicited significantly higher frequencies of intrasentential code-switching. Moreover, the qualitative results showed that the most frequent English words within Arabic sentences were technical and academic terms, whereas the most frequent Arabic words within English sentences were religious words. Overall, this study shows that the use of intrasentential code-switching among Arabic-English female friends on the social network Facebook is a natural part of their interactions and the frequency with which they employ code-switching is related to the topic of their communication, their language environment, their cultural experiences, and their religion.