1996 some AOL chat room. Shortly after that I found some primitive archery groups.
There were a couple of small (and private) forums but the first major one was the James Randi Educational Foundation (JREF) Forum. Made a lot of friends there that I have to this day. It was dropped from the JREF site some time ago and I’ve pretty much given up on it’s successor, spending most of my time here on the SDMB.
I was on Usenet back when alt.sex was a single low volume group. That was not my community, I’d say more alt.atheism for participating, talk.origins for lurking and a bunch of others.
However I was also on a Mike Oldfield mailing list, one of the few Americans on it.
I was going to Madrid for a business trip, and I asked about places to get Mike stuff there. In one of my first experiences with the power of the net, I dragged my colleague to a small record shop on a small street in Madrid and got a bunch of bootleg CDs (rare stuff, not copies of legit albums.) He was amazed.
My first real community was on PLATO, but that was pre-Internet. That was my first message board and chat room experience, not to mention multi-user games.
I think it was more a mailing list rather than something you posted to, but it was for help with the typesetting program TeX. It was run by someone at the Sam Houston Institute of Technology (think about that name for a minute). It’s successor at texhax@tug.org still exists. I can’t recall when I started using it, but I started using TeX around 1982 and got into hacking it in the late 80s. TSD was my second. I discovered it when our local throwaway paper dropped Cecil and I was looking to find an online version.
AOLs old Civil War Chat Room and a BBS for muzzleloading shooters followed shortly by the original Ecunet. I never got into message boards too heavily (maybe 3 others over my life). This place is very much the exception for me.
CompuServe Apple Forum in 1986.
I also ran a Flight Simulator BBS on a 286 with a 1200 bps modem.
I remember accessing bulletin boards in the mid-80s through Prodigy, made a few friends across the country exchanging recipes. It was powerful stuff.
Prodigy led to Compu$erve CB chatrooms. I became part of a large (for then) online community. Compu$erve charged by the minute in those days. I endured many obscenely large phone bills, but also created a lot of wonderful memories of hilarious online interactions and face-to-face cons around the world.
The community left Compu$erve after they refused to move to a flat rate pricing system and moved to IRC for a few years.
The group now has an active Facebook group. I don’t do Facebook, but I’m still in touch with quite a few of those folks via phone, email and visits.
It took forever for me to stop inserting <g> and <s> in my online interactions.
There was a BBS Tolkien group that I was part of in the early 90s. I got a rep as a trivia* king. The community must have numbered in the dozens. Maybe even a gross.
[SIZE=2]* or perhaps trivial[/SIZE]
One Gross, indeed! Vulgar expression.
Here.
Some of the rec.alt.startrek groups in 1995-96.
Either here or one of a couple of different webcomic comment sections. (I can say that with confidence because my internet presence is still limited to here and webcomic comment sections.)
AOL in 1994.
Also did IRC and newsgroups, and I’ve had a Yahoo email addy since around 1995-96.
Been an Amazon customer since 1997. 
Yep.
mainly aol stuff i joined about aol 1.5 …… mostly rp rooms and neverwinter nights original version
then I discovered video game emulation and was on some of those boards
I tried mirc a bit
but here and game faqs are the two ive been on the longest and I quit here for about a decade …….
The Science Fiction Round Table on GEnie. Was given a free account in 1989.
I also spent time with FidoNet’s science fiction groups.
It must have been a Star Trek-related mailing list (remember mailing lists?) called “strek-l,” back in the early 90s. I was a pretty prolific poster there back in the day (under my real name!), and pretty well-known within that community. There were sometimes upwards of 100 posts a day.
It was stable for a long time, but then for some reason it lost the site where it had been hosted, and started migrating between various host sites. That, plus the decline of mailing lists in general, led to a steady decrease in membership. It held on for a surprisingly long time, the daily number of posts dwindling and dwindling, until they just petered out. I don’t know if it ever officially ended, or if everyone just stopped posting to it. Either way, it’s long gone now.
CompuServe, circa 1991. I mainly signed up so I could upload the game I wrote for my senior project as shareware. After joining, the main place I hung out was the Comic Books forum…is was a really great “place” to be for a comic book fan, many comics professionals participated there, including Mark Waid and Neil Gaiman.
For me, there were BBSs all linked by Wildnet and Fidonet long before I got access to Usenet.
alt.fan.monty-python on usenet and the telnet “talker” (or social MUD) that sprouted from that community.
My username originated from being Sir Not-Appearing-In-This-Alphabet there, since the ascii world was not kind to Norwegian characters and my real name has an ø.