Vbulletin boards and general forum sites before the likes of Facebook and Reddit got big. Made a lot of friends on there.
Neopets was great as well. Loved the minigames.
I admin-ed my own vBulletin board around 04-06. The script itself cost about Β£90 back then. It was so great though. Small community of like-minded people all chattng. It was like a slow-Discord or a more intimate Reddit.
I installed a ton of extensions for people to play Flash games at one point too.
One day I accidentally forgot to pay hosting and the site vanished. Lost it all. Never bothered relaunching.