For example, I started 20 years ago with GameFAQs and AIM. Had a bit of a gap in text-based mediums between AIM and joining Reddit in 2010. Still engaged vocally in private servers in video games like Counter-Strike during that time. Sometimes that leaked into Ventrilo. Actually made some legitimate online friends that i engaged with for at least a few years with that. The popularity of ranked matchmaking eventually killed casual conversation in that medium for me, though. Stayed on Reddit for a long time, still follow some specific subs. Started on HN about maybe a year ago
Tried discord here and there but it doesnt seem to stick to me
I was on MySpace & Facebook early but didnt really engage with strangers there, just IRL friends and such.
BBSs and Fidonet, Usenet, early web forums, ICQ, AIM, Jabber, Mailing Lists (so many!), IRC, Friendster, Orkut, FriendFeed, Blogs and their comments, Convore, Hacker News, LinkedIn, Twitter, Quora, Github, Goodreads, Gchat, Hangouts, Google+...
I probably forgot a few.
Notable absences (so far): MySpace, Facebook, Discord, Twitch, Tumblr, Instagram, Second Life, Snapchat, MUDs, Stackoverflow, MMORPGs & games in general.
BBS irc/forums Counter strike/subspace MMOs Reddit Twitter Discord Slack