I've been thinking about this quite a bit. I'm friends with a couple of people I've connected with*, but very few. And I'm also most definitely NOT friends with a couple people I've connected with. But most of them I'm just not in touch with. I don't harbor any animosity towards them, but we aren't friends.
I don't know how much being poly has to do with this though. I didn't identify as poly until relatively recently (I agreed with the concept but wasn't familiar with the word). MC and I were semi-open in our marriage but it was all FWBs, no serious LTRs. So there were rarely "break-ups" in the usual sense of the word. Usually the "benefits" part would end BECAUSE the friendship ended. So staying friends really wouldn't have made sense.
Also, I realized that TGIB is the first poly person I've had ANY kind of connection or relationship with. No one else (as far as I know, at least) was looking for or trying to have a serious relationship with me. So perhaps their more "mono" mindset also contributed to us not staying in touch afterwards.
*I could use the term "relationship", but to me a relationship implies some sort of commitment.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack