This study aims to gain a better understanding of how the newly arisen social messaging may impact the practice of peer assessment. Seventy-nine ESL (English as Second Language) students reviewed each other’s English essays in three peer assessment groups: a three-member group using wiki (wiki group), a three-member group using social messaging (small messaging group), and a six-member group using social messaging (big messaging group). Data analysis suggested that peer assessment facilitated by social messaging can be at least of the same effectiveness as wiki-facilitated peer assessment on ESL students’ writing skills and intrinsic motivation. In addition, the findings indicated that students in the small messaging group outperformed students in the big messaging group on essay writing, and reported a significantly higher rating on Perceived Competence, a positive indicator of the behavioral measures of intrinsic motivation, than students in the big messaging group.