Reddit is the new usenet.
Use the search engine. If you type in "polyamory" somewhere it will put up the link to the subforums and you can add it to your favorites.
It seems rather poorly organized to me too, and there is a ton of stuff i don't understand. I just go there to read Sexual Accomplishment Sunday, a recurring thread in /r/sex, and to see what kind of wacky threads are "rising" and "hot".