The present study examined students' perspectives on using smartphones and social media to fill a lacuna in mobile-assisted feedback in L2 vocabulary learning. As part of a vocabulary-building activity, 208 undergraduate students in Korea drafted sentences incorporating target vocabulary taught by their instructor and uploaded their sentences to a social media forum using smartphones in class. They then received immediate teacher feedback on appropriate use in context and accurate use in grammar. Near the end of semester, the students completed close-ended and open-ended surveys. The survey results revealed four benefits of this kind of feedback: (a) Induce Comfort and Active Learning; (b) Increase Interest and Satisfaction; (c) Enhance Cooperation and Sharing; and (d) Improve Word Use. Five constraints were also reported but these earned very low mean scores, suggesting they were not serious problems. The open-ended survey questions discovered that technology played a crucial role in receiving timely high-quality feedback, and keeping students engaged. Implications for teachers regarding L2 vocabulary feedback are discussed: (a) the importance of providing timely feedback by utilizing various technologies; (b) being aware of the benefits and constraints of these technologies; and (c) developing strategic teaching methods.