While social networking sites (SNSs) have attracted growing scholarly interest in uncovering L2 learners' participation within these sites, the impact of writing on SNSs upon students' writing performance and learner autonomy remains underexplored. This study reported on a college EFL writing course participated by two groups of Chinese first-year English majors at a university in Southeast China. The students were arranged randomly into an experimental group enrolled in a writing on WeChat Moments program (n = 25) and a control group in a traditional English writing class (n = 25). Both groups took part in the pre- and post- writing tests and completed questionnaires. The results revealed that the participants in the experimental group achieved better writing performance, with higher levels of learner autonomy than those in the control group. The findings call for attention to the significance of WeChat as one SNS in rendering richer exposure to the target language resources and constructing an interactive environment of learning and social communication for L2 writers.