Social media have become more and more popular all over the world, especially among adolescents. Using social media to teach English is not new and many studies have been conducted to explore the advantages that social media can bring to foreign language classrooms, especially on productive skills such as speaking or writing. Very few studies, however, have investigated one of the most popular means of social media: Facebook and its effects on students’ writing skills. This study will respond to this gap by investigating whether peer feedback on Facebook can help students improve their writing performance. 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. The study took place in one high school in Vietnam. Participants included 39 students (21 females and 18 males) of Grade 11 and four English teachers (2 females and 2 males) responsible for scoring ten model papers as well as the writing drafts. 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.