First serious one was, in all places, on Battle.net in Diablo 1 and continued for a few years on Diablo 2. It was a strange system with terrible chatboxes, but the adventure was born, and the first group I ever did anything with was created. Said group I was involved with for a long journey before disbanding a little while before I joined the guild.

The very first one was where I was introduced to a DnD board. I looked at the dice and went "nope". I guess that was my first experience. >.<