Abstract-Facebook (FB) is a social networking service with more than 1.23 billionmonthly active users all over the world. Millions of Iranians use this website and although it is possible for them to write in Persian, many of them prefer to write in English even when they communicate with their Iranian friends. Also, majority of them use the English interface of the FB rather than the Persian interface. This study investigates cultural and psychologicalmotives behind Persian speakers' preference of English over Persian. For this aim, 180 Iranian FB users answered to a questionnaire asking about different aspects of use of English in FB. Then, 12 of them participated in a semi-structured interview.After coding and analysis, findings of the interview were used to add to the findings of the survey. Results indicated that Iranian users are willing to post dialogues from famous movies and scientific content in English rather than Persian, but prefer to discuss their feelings in Persian. In addition, it was found that majority of Iranians believe that writing English in FB can be interpreted as an indication of higher social class, education, and prestige of the users.