What to call this stuff...
Jokingly:
webweb, 'cause it's a web of websites.
cross-fora, 'cause they're like cross-posts but entire forums.
newsvents, 'cause a lot of the activity is venting in news post comments. ๐
memecycling centers, 'cause it's a lot of reposts of old memes/shitposts.
Realistically:
Whatever instance/site I'm directing someone to.
stick-in-the-mud-tangent
I'd never tell someone to go to WordPress if I was telling them to go to a site built with Wordpress, that'd be silly and out of touch. It'd also be out of touch to use some jargon that means nothing to them like "threadiverse".
All the pointing to these backends as though they're platforms like the corporate platforms shows how much they've conditioned people into thinking in their terms. The major benefit to these backends is they're more open, enabling greater mobility between the instances of them/sites built with them.