It was 2002 for me, some sort of RPG Maker forum that I've long forgotten the name of. I did make a couple friends there, but the most quintessential forum for me was Fated Circle. Another RPG maker site, it seemed to be fairly large and used the coveted vbulletin software. I joined there in 2003 and made a bunch of friends that I'd regularly talk to on messengers, some of us would play Diablo II or Ragnarok Online together. Lost touch with pretty much all of them save one but those were fun times. And of course we'd make our own crappy forums with barely any activity, that would use whatever the free alternatives to vbulletin were.
It's interesting to look back and see the different phases of internet participation. In the late 90s and early 2000s, anyone could have a free basic Geocities/Angelfire/etc. site, so many did. Then a bit later it was the same with forums. Then came Myspace pages. After that I don't feel like it counts as much, Facebook and Twitter don't let you personalize your page/section at all beyond links and brief descriptions. Maybe Discord servers are somewhat of a return to form in that sense, but you still are very gated on how you can customize them. The internet and these portals where we are encouraged to scream every thought into the void just feels so sterile.
To me, a forum is the perfect middle ground. It's anonymous if you choose, but you still see avatars and signatures, so you get to know people more than you would in a reddit community. You can personalize and customize what people see with your posts, but it's not something that can mess up the whole viewing session, like going to a Myspace page where there are 4 songs playing at once and a bunch of glittery blinking GIFs lagging your browser. And most importantly, if someone gets out of hand, you can count on a moderator taking care of it, unlike Twitter or Facebook, where no one is willing to turn off the sewage pipe. Also, you just leave when you want to and it doesn't follow you with notifications on your phone. Maybe if you enable email notifications, but that's it. I think that aspect of being able to disconnect is healthier than other social media.