This research investigated if and how the instructional design affected the learning outcome in a blog-mediated project, namely students' communicative writing in the social media. Thirty-four college-level English learners from two universities in Taiwan participated in the telecollaboration. The current research continues to highlight the social networking nature of Web 2.0 by locating idea units (IUs) in students' semi-formal weekly discussions, instead of focusing on syntactic complexity of conventional written modality. In this study, the language learners' output performance (the quantity of IUs was operationalized as the dependent variable) was hypothesized to be affected by task type and duration time. The findings indicate that both were significant in eliciting the amount of IUs. As a result, the contributions of this study include (a) shedding some light on the academic and educational capacity of blog-mediated telecollaboration, (b) revealing multiple factors that might affect the telecollaborative mechanism with the use of weblogs for language learning purposes, and finally (c) evaluating interactive blogging activities' educational value for complementing formal-writing training.