Social media have become more and more popular all around the world especially among adolescents and this is not an exception in Vietnam. Studies have been conducted to explore the advantages that social media can bring to foreign language classrooms. However, few studies investigate whether peer feedback on Facebook can help students improve their writing texts in high school contexts. The current study aims to find out (1) the effects of peer feedback on Facebook on EFL students’ writing performance and (2) students’ attitudes towards the use of Facebook for peer feedback. This experimental study followed a one-group design using students’ first drafts and final drafts of two writing topics. Participants included 39 students (21 females and 18 males) of grade 11, four English teachers (2 females and 2 males) responsible for scoring ten model papers as well as the writing drafts and the researcher as the class instructor. Data collected from students’ writing scores and questionnaire indicated that students’ writing performance increased after the study and that the majority of them had positive attitudes towards the use of Facebook for peer feedback.