For me there’s a bit of a network effect where the polycule sprawls out into the distance. Partners have partners who have partners.
But for disconnected folks, it’s mostly been tinder (yuck), and a local meetup.
(Also this might be the first post? That or nothing federated yet)
Various caveats apply, ymmv, etc. There is a significant overlap between Poly, and pagans, furries, kink, leather, gamers, programmers, queer folx in my experience. I suspect others could add more demographics to the list. I’ve found that going to pagan gatherings has been a good way for me to meet like minded people, which can evolve towards relationships of various sorts. I’m not much one for dating apps, I could tell you a few, short, unsuccess-filled experiences. But that’s me, and I’m sure if I put some more energy into it, other results would accrue.