This study explores the positive learning effect of formulating English sentences via Social Network Service (SNS; "Kakao-Talk") on less proficient L2 university students' (LPSs') writing, when the application is utilized as a tool to link in and out-of class activities in a multimodal-learning environment. Its objective is also to promote LPSs' participation in writing activities as well as to reduce their anxiety about potential mistakes through offering writing opportunities in a virtual space. The LPSs (N = 62) were regularly directed to construct their own sentences and opinions via "Kakao-Talk" in accordance with an instructor's specific "to-verb" prototype after receiving the prepared materials via "Kakao-Talk," which stimulates the "recalling process" on the previous class content. Qualitative and quantitative methodologies including correlation and corpus analysis were conducted. The results support the benefit of correlated factors: the efficiency of input, challenge, social presence, the usefulness of "to-verb" practice, and feedback. In particular, multiple-regression analysis presents a highly workable model to maximize the efficacy of learning. Multimodal Teaching and Learning with an SNS-based writing platform can have a significant effect in fostering LPSs' writing performance through a specific grammar-driven ("to-verb") approach, invoking learners' motivation and enhancing peers' collaboration as a newly-developed learning venue.