Interesting observation. I grew up with shitty (but fun!) phpBB forums and the fancier vBulletin, but wouldn't categorize them as 'social media'. The first of those were probably cu2, MySpace, and eventually Facebook/Twitter/etc.
When I think about it, it seems like a somewhat arbitrary distinction to make. I suppose the main difference is that the forums were self-hosted and I could have different identities for each one. And while I generally reused the same username, the fact that it wasn't just a matter of 'clicking on my profile' to see all my activity was something I really liked.
Maybe to me the term "social media" has come to mean "social media /companies/"? That doesn't quite satisfy, but it really does feel like the primary difference is that on these forums I knew that they were generally not run for profit, and the 'gods' were just individuals who volunteered their time to host or moderate. It felt safer, and more personal.
For those us whose Internet citizenship pre-dates Friendster and MySpace, the term "social media" has a particular meaning.
But for many younger "social media natives", the term basically just means "any website or app where you can make posts or comments".