As social media expands exponentially, such technology is proving to have the potential not only to transform approaches to teaching and learning but also, and more importantly, to transform learners themselves. Meanwhile, current education scholarship is calling for a new paradigm where learners are seen not as information receivers but as 21st century critical thinkers with the well-developed skills to communicate those thoughts. Yet, this discourse often fails to provide practical examples of how to arrive at this paradigm and surprisingly, reflects an unawareness of technology's possible role in enabling this process. This disconnect is particularly counterproductive when the learners are our future teachers, the very individuals that will soon be responsible for fostering those 21st century skills for the next generation of learners. Our research seeks to close that gap. We used a Qualitative Longitudinal Case Study approach to examine peer mentoring of oral communication skills among pre-service teachers in the context of a Chilean private university's English as a foreign language (EFL) teacher education program. The focus of the classroom-based study was on the responses of future teachers to the audio files of their peers' oral communication in an online community blog and the extent to which they reflected critical thinking behaviours. We draw on sociocultural theory, and its emphasis on agency and investment, to explain the transformations of these individuals in terms of critical thinking skills and their identities as future teachers. The encouraging findings from the significant and rich data collected in this study clearly indicate the value of leveraging social media at the classroom level to enable critical thinking through peer mentoring, while enhancing participants' knowledge construction. We consider that a multidisciplinary application of this approach involving social media can offer hope to those struggling with how to promote 21st century transformations in educational contexts.