Going back 25 years...
1998 - the Virgin.net noticeboard (so many trolls!)
1999 - Montypython.com (as in Monty Python)
2000 - Napigator (Napster plug-in)
2002 - 4clubbers (clubbing)
2003 - Sundissential (clubbing)
2004 - Overclockers (here!)
2007 - UKRides (fairs / theme parks)
2008 - ATFOTF (fairs / theme parks)
Only Overclockers and ATFOTF are still going and I still post on both.
People here have mentioned Reddit. I opened an account in 2018 and occasionally post on there. I find it more interesting than the usual Facebook/Twitter mob.
I also joined a Disney fanart forum in 2010 and it was so strict that I (unintentionally) broke the rules countless times. The art that you drew had to be uploaded to the forum itself, you couldn't link to your deviantART profile etc. In the "introduce yourself" section, you couldn't say your real name or put your age /location in your forum profile. Any discussion about or linking to other Disney sites wasn't allowed, including the official Disneystore. Then the username you picked had to be formed of 2 words put together from a legal list of 2 drop-down menus. Both drop-downs had around 100 words each, so around 10,000 possible combinations and the combination I chose got modded by an admin and I had to pick another! I couldn't be doing with that so only lasted a couple of months. The weird thing was, that although I wasn't allowed to put my age in my profile, I still had their automated "happy birthday" email every year for the next few years lol