Can include ones you took a hiatus. The longest one I was a member on was my own 15 some years ago. It went for over 3 years. Otherwise, any forum I’ve been on since, the activity lulled enough for me to stop going & delete my account.
16 years I was a member of GameWinners.com, ran the community side of it for nearly 5 years, I hated the day the owner shut it down, because so many of the members literally grew up with that site. Some even met and married because of it. Such a special site and community it was.
Been a member on lcpdfr.com for 10 years that's still running, other sites back then have closed. But many sites that I can't even remember what I have joined for 😛 I can say YouTube is far longer when I made the account in 2009.
MajorGaming is another long term member that is no longer running, I join in late 2012 and after 6 months later became staff and lasted for while till 2014 when the owner is just gone.
Between account deletions and remakes, I have been a member of ProBoards Support, since May 30, 2001. If swirve.com didn't sell off it's games, I would still have a forum account with them, dating back to 1998.
My first ever forum was an anime one. I've been on that forum since 2008. But it's mostly inactive. Every now and again the members will get together and start talking about upcoming movies and old anime shows.