This excellent question came up in class on Tuesday. It turns out the answer is pretty flexible, and as long as you tick off a few necessary boxes–internet-based, involves personal interaction and conversation, etc.–you can probably find a way to make just about anything count.
To give you a sense of how wide-ranging and specialized social media can be, here’s a non-exhaustive list of possible social media websites/apps: http://traffikd.com/social-media-websites/
And to give you a sense of how easy it can be to think of it more narrowly, here’s a definition of what social media is not (only): http://brandonsutton.com/facebook-is-not-social-media/
Any other boundaries you might place on what should count as social media? Any strange or unusual cases you’d like help puzzling out?