Although the use of social networks, especially Facebook, has become a common practice among educational communities in Egypt, studies have yet to explore the online practices of in-service (language) teachers as part of Continuing Professional Development (hereafter CPD). Therefore, this study investigated English language teachers' informal online professional development that may include some types of language learning for improving their pedagogical content knowledge. It also explored differences between teachers' perceived use of Facebook as a venue of CPD. This study employed a mixed-method design, collecting qualitative and quantitative data via content analysis and a questionnaire. The participants were 180 English-language Egyptian in-service teachers. The qualitative analysis of teacher's posts demonstrates that CPD informal groups, as maintained by Egyptian EFL teachers, display different aspects of professional knowledge. Results showed that the data yielded seven main categories: General Pedagogical Knowledge (PK), Content Knowledge (CK), L2 Pedagogical Content Knowledge, Knowledge of L2 Learners, Knowledge of Educational context and Knowledge of (professional) self and miscellaneous topics. Besides, ANOVA's Welch test indicated significant differences in the means of the sum scores of the questionnaire as well as two sub-domains (PK and CK) among primary, preparatory and secondary teachers in some aspects, such as pronunciation and vocabulary. This study concludes that CPD Facebook groups provide wide options and spaces for continuous learning and informal study for EFL teachers. Pedagogical implications and recommendations for further research were suggested.