As most traditional classroom environments in English as Foreign Language (EFL) still restrict learners’ collaboration and interaction in college writing classes, today, the majority of EFL learners are accessing Social Networking Sites (SNSs) as online communities of practice (CoPs) for adopting informal collaborative learning as a way of practicing English beyond the classroom. This study aimed to investigate the opportunities and challenges of SNSs as learning environment in writing in English. The study was conducted among 24 active and regular EFL learners joining the Only for English Learning Facebook (FB) CoP – a group developed and maintained by a few instructors in English – for EFL learners coming from different EFL Arab countries. The data was collected from the learners’ interactional exchanges in the weekly posted writing activities as well as their responses to online open questions posted by the instructor. Based on the mixed analysis of the data, the quantity of the EFL learners’ participation in the writing activities highly increased in the second session. Moreover, the learners were motivated to generate ideas, write their paragraphs and scaffold each other in paragraph writing. The findings also revealed that the EFL participants perceived this CoP as an interactive learning environment that contributed to enhancing their writing by engaging in learner-learner and learner-instructor interaction, information sharing, communicating and socializing with friends and developing a sense of belonging. However, a few challenges faced by the participants in such an online CoP were identified by the participants, and therefore, some valuable assistive features are suggested to be involved in the FB CoP for achieving further EFL development in the future.