While a great deal has been published about the educational value of social networking sites (SNSs) for teachers, less has focused on teachers who are arguably more dependent on these virtual platforms for informal professional learning due to financial constraints and a lack of regular in-service professional development. Situating this study in Vietnam and focusing on teachers teaching English as a foreign language, we examined how these teachers have adapted SNSs, a tool typically used in leisure and social life, into online personal learning networks. The evidence was collected from 393 teachers' responses to an online survey. The results from descriptive and non-parametric inferential tests revealed three main findings: (1) Teachers engage in a variety of professional learning activities using SNSs with high frequency, regardless of their self-perceived levels of technology competence or teaching experience; (2) Teachers make use of online groups on SNSs for professional topics and their preferences of topics are not likely to be determined by their experience, except for the topics of classroom management techniques and knowledge about learners; (3) The most decisive factors these teachers associated with effective professional learning on SNSs include time availability, reliability of information and materials, personal information privacy and desire to learn. Teachers' perceptions about these factors are influenced by the frequency of their SNS use and professional work experience. The implications of these findings for teacher professional development in Vietnam, and other countries in similar economic circumstances, are also discussed.