This study investigates a Saudi tertiary EFL (English as a foreign language) students’ experiences and use trajectory of WhatsApp for their out of class English learning in a longitudinal study for a 12-week semester. The study utilized agency theory (van Lier, 2004, 2008, 2010) as the analytical tool to acquire a deeper understanding of how the students’ use of WhatsApp could introduce innovative ways that could assist them to practice and collaborate in English learning outside the classroom. A mixed methods approach was adopted for data collection and included six instruments consisting of initial and post-project interviews, focus group participation, a post-project questionnaire, observations, and research log to gather data from the students’ use of WhatsApp. The participants were 25 male students who are studying in the first and second levels of the Preparatory Year Program (PYP) at the main campus of Qassim University. The study results showed that WhatsApp’s implementation demonstrated its sustainability outside the classroom for English learning as evident by the students’ continued use of WhatsApp even after the study ended by the 12th week; in fact, they continued to use it until the end of the semester (week 17). The affordances of WhatsApp allowed the students to develop autonomy, to practice what they were learning, to collaborate with each other, and to shift from being passive learners to active ones, not only in English studies but in other subjects as well. Finally, this study concludes by demonstrating the significance of interpreting the dynamic and complex nature of the innovation’s implementation process through the lens of van Lier’s agency theory from an ecological perspective. It also identifies the limitations of the study and recommends ways in which researchers could investigate the advantages of social networking in the language classroom.