This study investigated the effects of Facebook collaborative writing groups on ESL undergraduates writing performance. A total of 33 second year ESL students were involved in this study. The students were divided into two groups; control and experimental groups. Facebook, as the treatment in this study was compared to a conventional method; face-to-face in the collaborative writing activities. The comparison between face-to-face and Facebook collaborative writing groups were made in order to measure students' writing performance on the intervention based on Jacob's et al. (1981) ESL Composition Profile; content, organisation, vocabulary, language us and mechanics. Before using this approach, a pre-test was administered to all students and based on the preliminary results; students were divided into Facebook and face-to-face collaborative writing groups. After using both approaches, a post-test was given to participants in both groups. The data were then analysed using inferential statistics; independent t-test and paired sample t-test. From the findings, it was found that participants in Facebook collaborative writing groups displayed slightly higher scores compared to face-to-face collaborative writing groups. However, the differences between Facebook and face-to-face collaborative writing groups were not significant in the post-test writing scores. When comparison was made within each group, this study found that there were significant differences for overall writing performance, content, organisation, vocabulary, language use and mechanics. Meanwhile in the face-to-face collaborative writing groups, there were also significant differences for overall writing performance, content, organization and vocabulary except for language use and mechanics.