Given all the arguments above this post, I don't think there's a lot of value in trying to categorise any particular website as a yes/no to "is this social media". All that achieves is people trying to litigate whether a site fits a definition nobody can agree on.

Much more effective is trying to identify the mechanisms by which a communication platform breaks social interactions. Is the feed sorted by engagement or chronologically? Does the platform encourage you to chase metrics? Does the default feed include content you didn't subscribe to? Are comment threads difficult to trace through?