As far as I can tell, Mastodon was briefly hyped on HN but nobody actually uses it. Bluesky seems to have a few people within a fairly narrow political range. Truth social is just for Trump. Reddit is pseudoanonymous as is HN. Instagram is for sharing photos not ideas or links. TikTok is a Skinner box.
I ask this as someone who genuinely doesn't know how to use the internet anymore. Reddit used to be useful but is now a cesspool. LinkedIn is a weird place where we all post like Stepford wives for our employers. The twitter-clones all feel a bit like using a paper straw to fight climate change.
I know there are semi-private slack groups and discord channels out there, but I don't know how to find or join them and it seems like a hassle to follow.
Basically, for me, no one I pay attention to posts anywhere any more.