This is an interesting idea, or maybe we just need people to be more willing to comment in old posts. Reddit is actively hostile to old posts, but they should work fine in Lemmy/PieFed since we have sort options like Active and New Comments. On old forums people used to continue talking in old posts forever, we could do that here no problem, the software supports it.
People may be cutting back on Reddit, but they do not seem to be bringing those discussions here. Perhaps people are discoursing more IRL rather than social media.
Or Facebook, Facebook groups are massively popular
I think this could also be improved with a better search page, which shows how active the communities are in the search results so you don't have to click through them all, I think v1.0 is getting that
So last year was 15%? That's 1 percentage point, but you could also say it's down 6.7%, although we don't have precise numbers to work with