According to Swain, Kinnear, & Steinman (2011), we use a language with others as a form of shared cognition, and in the process we scaffold each other. This action research investigates how students' online written output affects each other's writing. One thousand twenty online entries written by 21 Japanese university sophomore English majors were collected and analysed, specifically focusing on changes in two linguistic features: subject-verb agreement, L1 use and variant L1 spelling in L2 writing. First, all 21 students accessed a specific Social Network Service (SNS). For two months, each student took turns offering a discussion topic with a minimum of 150 words, and the rest of the class members commented online with a minimum of 20 words. The task resulted in 54 topic strands. Each student was tracked to see if his/her language use reflected the output of others. Then the linguistic developmental patterns were further investigated in a post-treatment interview. It was discovered that students lacking confidence in English learning are less likely to imitate and internalize from others. The study suggests that, in addition to scaffolding provided by peers, positively affecting the Zone of Proximal Development (ZPD) is closely related to affective domains that give rise to particular identity formation. This paper therefore argues that the extent of languaging is significantly influenced by affective factors.