Special Interest Groups and Communities of Interest.
Especially with long form text and commentary.
What's the best platform for these in 2018? IRC, Usenet, Skype Groups, Yahoogroups, Mailing lists have all died. Most phpBB/vBulletin sites have died. G+Communities and GoogleGroups are f*cking useless. Reddit works but the geeky barrier to entry in the UX is too much. IMHO, right now Facebook is the best with the most engagement. Though it too has problems.