Fediverse

And just to add some more detail to this.

For twitter -> mastodon ... there are alternatives beyond mastodon like akkoma, calckey, misskey, pleroma (where akkoma came from). From what I can tell, akkoma is cool, but mostly used for hosting your own single-user or small user-base server, but not exclusively at all (for instance, the linux kernel developers run their own instance for themselves on akkoma ... Linus Torvalds is there and occasionally posts). Calckey is fun and feature rich but also being actively developed.

For Reddit -> Lemmy, there's also kbin, which is newer, took some inspiration from lemmy, but is fusing microblogging with threaded posts, kinda cool and I'd like to see kbin and lemmy grow together.

Pixelfed seems to be doing well with a committed developer and an app and a newly released feature to import your instagram history.

If you're into blogging (remember blogs!?), there are some options too. Lemmy could be used by just starting a community and allowing only moderators to post (this is recommended in the documentation). Otherwise, writefreely and micro.blog provide some platforms while microblog.pub is an interesting platform that you would have to self-host but, if you have the know how, may be pretty suitable for hacking it to your liking (python + sqlite).