This study investigates the impact of Perusall, a social annotation tool, on an online postgraduate course conducted over two semesters at an Australian university. We examine the connection between students' pre-class engagement and learning outcomes, utilizing both secondary data from Perusall platform and primary data through a survey. The findings indicate that pre-class social annotations have a positive impact on students' performance on post-class assessments. Notably, English as an Additional Language students with low English proficiency achieve comparable results in Perusall as those with high English proficiency. Additionally, the study identifies key aspects of social annotation that students highly value, providing insights for future implementation. Overall, this study highlights the potential of social annotation tools like Perusall to improve pre-class engagement and enhance learning outcomes.