October 26 2002, on Forumplanet (specifically the Deus Ex and Warcraft boards). The date is pretty much a second birthday to me tbh. The place had a lot of secret boards (thanks in part to our very own Baroque for cataloging many of them), which facilitated my acceptance of pornography and posting random shit for the sake of it. December 22 2002 I joined Nationstates which is where I started to really feel confident enough to express all of my various bullshit opinions. It was also the first board I hit 10,000 posts on and the first one I was banned from. Anandtech followed a bit later in 2004 and was kind of my main home until I really started to get an interest in discussing music, beginning with the official Iron Maiden board in late 2005 or so, and then the off-shoot MaidenFansUnited when they got greedy and wanted their fans to pay to post on their forum. People on MFU introduced me to a lot of metal not in my dad's collection, which led to finding Metal-Archives and many other metal forums (I believe this was the first one I registered on aside from M-A). When I started to branch out a little more I started to post on RYM, where I spend a decent amount of time. Forums in general have gotten so slow that starting late 2013 I've begun posting on 4chan, which I have to admit is highly addicting when considering how quickly a discussion can end, knowing it won't be there for me the following day. I'm nervous about what the future may hold, and what I will use for a social outlet if everything turns towards social media.
EDIT: The Snopes forum was the first forum I ever read/lurked, though, iirc, probably around 2000-early 2001. Fond memories of very cheap photoshops of human breasts the size of beach balls in the 'NSFBSK' section, people asking "ARE THESE REAL?!?!", and touching myself to them.