Language learners are actively engaging in language learning beyond the classroom. However, social networking sites, despite playing a major role in people's lives, have been found to be rarely incorporated into language learners' learning ecologies. An understanding of the factors that shape learners' engagement on instruction-oriented social networking sites could inform platform design and enhance the likelihood of platforms being utilized. This study examined a group of language learners' engagement on an instruction-oriented social networking site, Lang-8, over time. Using narrative data and learners' behavioral data on the platform, the study revealed how various ecological resources on and outside the platform interacted with one another to shape the dynamic changes in different dimensions of learners' engagement on the platform over time. The study also suggested that learners' engagement on the platform further induced reconstruction of their language learning ecologies, providing additional learning opportunities both on and beyond the platform. The findings highlight the importance of supporting learner engagement on technological platforms in an informal learning context and provide insights into how such support could be achieved through system design.