I'm curious, why don't you consider TB to be social media? I would say it is. It's certainly a medium - information gets transmitted through it. And it's social; there are members, all members are able to post here, we can react to each other's posts with likes and replies, we can message each other. That's how social media works.
I would say, thought, that TB is a certain KIND of social media; an interest-based private forum. It differs from the big social media platforms in that it doesn't aspire to be ubiquitous for everyone on the planet, but only for the community of bass players, and as a result - critically - it doesn't have an algorithm to generate more "engagement." Also, since it's private and interest-based, it's much easier to establish moderation rules and enforce them; that allows us to keep the discourse here civil and on-topic.