I used to have a go-to answer for this but I have since been able to experience it again: "City of Heroes".

It was the first MMO I ever reached max level on, the first one I had meaningful social interactions on, a game that inspired imagination in me. It was really special to me in the time that I had it and would always come back to it off and on. And then it shut down which was devastating to me.

Since then though private servers have become available and I tried it again. I liked the nostalgia but realized it hasn't kept up with where I've gone in life. I still treasure it, but now as a fond memory of those specific past experiences and what they meant to me at the time.

Idk, without thinking about it for very long, probably MySpace. MySpace was better than FaceBook because you had more customization over your profile and you could use whatever name you wanted, not your real name or even a real-sounding name, and the profiles contained a specific set of questions you would answer to better express yourself (and not boring things like "What jobs did you have" like on Facebook). And you could have custom colors, you could even have it automatically play a song when they visit your profile (I had it play the fragment "There is no / place I know / that compares with pure imagination" by Gene Wilder from Willy Wonka and the Chocolate Factory). And I had the feeling it was used more for dating purposes than Facebook is. It was how I met my second girlfriend, whom I still talk to today. In fact, she's going to visit me from Norway on the 22nd.

Oh, another thing that was really good was AIM. Best messenger application ever. You could select font size, font face, and font color (per character), you could log to a custom directory, you could transfer files and directories, you had a bio (where you could use custom colors, fonts, etc.), you had a pfp that could be an animated gif (I programmatically made a few for it, including an infinitely zooming dragon curve fractal or something like that, TV static, and one alternating between all black and all white at like 50 frames per second.) And it supported voice and video chat, and also chatrooms. And when someone messaged you, the window would pop up on your screen and take focus, which I liked, but Microsoft made it hard for programs to do that and they don't do that much anymore. I have a lot of memories chatting with friends on that. I even looked into running an AIM server recently (the application lets you enter a custom server address), but I decided there's no point because I only still talk to three friends nowadays that I used to talk to on AIM and I only talk to them every few days/weeks and everyone uses mobile phones nowadays anyway and AIM is desktop only. And the one I talk to most often can't even use a desktop PC, I think, because of her disabilities.