I think it is less about the hobby and more about the usage of forums in general.
Forums are in decline almost universally. It is not just this hobby seeing a decline in posts. Honestly, this site gets more posts/day than many forums do.
People are shifting to social media, mostly facebook. It sucks because I find facebook almost unusable. Especially since you can't sort the groups anymore.
As far as the hobby goes I can't really figure it out. It feels in sharp decline to me but the couple of good fish stores in my area are busier now than they were a few years ago. I suspect my feelings come from the fact that I am not invested in social media and so less of the activity is visible to me.