I've been going through this for a good ten years now. Been perusing and becoming involved with online communities since I first had an account on GameFAQS over 10 years ago, which I actually still have. Though at this point it's used to keep in touch with a couple of people on some social and private boards. Exhibit B is a browser based game I used to play the hell out of in my mid-teens, going from around 5k people playing to less than thirty over the course of a few years as console and mobile gaming picked up. That was a sad one, I checked in about a year ago and it looks completely abandoned at this point. Exhibit C, for me, is Xbox Live. Some buddies and I from high school became good friends with a lot of people from all over the place, but eventually people go to college, get married, go into the army, etc.
You have the memories and reminders from forum posts if the forum is still up, and maybe enough to keep in contact with people you come to care about in different ways.