The content was segregated and you explicitly had to seek it out. Not to mention, there was no algorithm to ease you into it, so even someone whose political views would lean towards a particular affiliation might recoil in horror at the craziness they'd see on one of those "specialized forums", where as Facebook will happily ease you in bit by bit until the craziness appears normal, even if you originally had no intention of reading about/discussing politics and just wanted to keep up with your friends' holiday pictures.
> is focused primarily on promoting the interests of one of a handful of relatively similar political parties over the others
I disagree. I believe the vast majority of divisive content nowadays is created & promoted by random people who don't benefit financially from it; in fact there's no single source (political party, etc) that would pay to originate this stuff, instead Facebook and other social media just use any divisive content to increase ad impressions, regardless of the political affiliation of said content. I'm sure political parties sometimes benefit from these "useful idiots" but even they don't actually fully control the narrative.