The purpose of the study reported in this article was to investigate the contribution of engaging in an online Teacher Professional Development (oTPD) course to improving different facets of English as Foreign Language (EFL) teachers' Professional Development (PD). To this end, ten EFL teachers of a language institute participated in the study to be taught via Telegram mobile app (as a common Mobile Social Network Site (MSNS) in Iran) during a six-month teacher training project. A 35-item technology-enhanced TPD checklist consisting of several main sections incorporating demographic items, background information, and different issues regarding TPD was designed to measure EFL teachers' self-report TPD scores at the outset and end of the training course. Results pointed to the positive effects of oTPD course on participants' PD across different facets. The obtained findings have some implications for teacher educators, syllabus designers and practitioners in EFL contexts to have a fresh look at the complicated construct of PD including several facets, consider using MSNSs in teacher education and TPD courses, prepare online teacher education courses based on teachers' needs and opinions and provide teachers with follow-up support by devoting an elapsed time between teachers' participation in oTPD and application of their learning in the course.