Having bounced around on social media a bit, it seems to me that a large part of the answer lies therein. Instagram is a great platform for sharing short trick videos without having to make a production out of it, and it ties into Facebook. Facebook also allows easy sharing of videos, but is more useful as a platform for discussion. A bunch of pros are on Facebook and contribute regularly.
I still love the forums, but social media fills some gaps left by forums. Forums do one thing significantly better than social media, though: they form a “catalog” of yoyo-related conversations. And ongoing conversations tend to be more persistent. Whereas a forum can sustain a “what are you throwing today?” thread, a new one comes up on Facebook whenever a user identifies that there’s not one currently being bumped.
All in all… since people want things fast and immediate, I think more are just using Facebook. And that’s enough “conversation” to occupy their time that they’re not spreading it as much onto forums. Me (not a pro), I see the value of both, so I use both.