Social media can benefit English as a foreign language (EFL) learning, but learners need a deliberate approach to pursue deeper learning. To this end, the aim of this study was to design a prewriting activity using a mobile-based concept map to assess participants' writing performance in different aspects of writing. The designed prewriting activity comprised (1) structuralized private knowledge, (2) explicit knowledge affordance, and (3) consolidated social activities. To evaluate the effects of the prewriting activity, an EFL prewriting experiment was conducted. In this experiment, 80 participants in a high school in Taiwan were evenly assigned to a control or experimental group. The participants' writing essays and learning perceptions were collected, and the analysis results indicated that (1) a significant difference was found in the aspects of communicative quality, linguistic accuracy, and linguistic appropriacy, and (2) the questionnaire statistics aligned with similar findings that the prewriting activity helped learners' writing from communicative quality and social collaboration perspectives.