Perusall is a collaborative annotation platform designed for pre-readings in a flipped classroom, but can also be used for stand-alone, asynchronous reading discussion components of courses. We examine the use of Perusall as a social constructivist learning tool in two upper year/graduate courses in Mechanical Engineering. Perusall was used to replace in-class discussion of readings during the shift to online teaching.  Data was collected from student surveys and from the student and instructor annotations themselves.  Annotations were coded for content, and examined for factors such as upvoting.  We found substantial engagement from students, with collaborative annotation providing opportunities for: correction of misunderstanding; linking concepts from the course and between readings; discussing larger issues around research and research writing; sharing background information among peers; and critically analyzing the readings. Students reported deeper learning than in typical in-class discussions of readings; however, they also noted that annotation required much more time.  Overall, collaborative annotation appears to be an effective method for course reading discussion.