In the repost on the Chile community, also saw one or other comment about Lemmy being toxic. I do notice a certain "toxicity", though from certain corners and which seems to stems more often from what I call "misery posting", which would be posts whose main focus seems to be to make the given topic appear like a lost cause to the reader (even some "meme" communities seem to fall on this).

Since some communities seem specially prone for such posts, I second blocking those as you notice the patterns to try to make your feed healthier, and to hopefully make such places shrink into a healthier size. Or at least, if you got the patience and resistance to mass downvoting and mobbing, to post in those communities and within their rules what to you is positive. Alternatively/Parallel to that, one could make sure to react accordingly to posts, instead of uninterestedly hide everything in feed as the user scrolls.

Otherwise, a given environment shrinking or tanking in growth could also mean people that use it are growing apathetic or anomic to popularizing it, to which I repeat my suggestion in the Chile community, change starts by small steps. So for example, if someone is on a platform compatible by extension but without an ActivityPub bridge or function active (e.g. Threads and Bluesky), to explain to them they could activate it, and e.g. Fedi Brdigy is working on Lemmy compatibility, so more potential users going around. Also, if you see a funny meme or the sort, you could share the link, provided it has a decent blurb preview on the platform to be sent.

Also spikes happen anywhere when competing platforms have issues, so current retention might just be its natural one.

And even on a Lemmy account I have (thus little noise from microblogging) and while highly curating my feeds there, the amount of posts is almost too much for me to be able to go through. So by observation, the "threadiverse" as a whole seems rather healthy in numbers.