Social media, if we consider a more literal definition of the term, is any media in which there is a degree of social exchange happening. Zheng et al., in their discussion of social media's classroom uses, use a more wide definition of the term too. Social media need not be limited to Facebook, Instagram, Snapchat, and Twitter, but can also be expanded to include any digital media that people interact with that has the capability to also engage users in social activity.
Therefore, a shared Google Doc that multiple people comment and collaborate on is social media. So would the comments section on a YouTube video, or a blog on WordPress. Expanding these definitions of social media within the classroom can also help students see a more wide application of social media in academic settings.
Learning platforms like Canvas, Blackboard, and Google Classroom, when allowing students to interact with one another, can also be part of social media. Of course, instructors can consider what limitations present itself through learning platforms but also the control that an instructor can have over learning platforms that might not be the case for websites like Facebook and Twitter.