Thank you for your comment @hilltop
specifically, I used the Instagram as the media for students to submit their work. I don't use it when I teach in classroom because it is hard to avoid students distracting. The reason I used this media is to promote autonomous learning for my students.
To deal with the limited number of texts, I suggested my students to write at 3-5 paragraphs in caption column, or the students can continue their paragraphs in comment column