I believe it was mentioned in a popular message sent out on the old WWWAC mailing list. I dropped in and it looked really interesting, so I signed up. I also remember this shift from BBS/mailing list world to Web World happening then, how nice it all felt.
On February 28, 2001, a big earthquake hit Seattle, I was working for a startup near Myrtle Edwards park (near the water, built on fill), and was really shaken by it, in more ways than one. When things calmed down, I went back inside, and my first thought was to go onto MeFi and see if anyone else was talking about it. The buzzy feeling of sharing stories, coming down from an adrenaline rush, contributing what I could... it was a fantastic feeling, a taste of interactions to come. It also made things a bit more personal, the site and experience felt less like I was chatting with anonymous usernames, and more like neighbors and friends.
Thanks Matt (and team now), for building one the best early community websites, and for keeping it rocking for so damn long - and to the community for not letting MeFi go down the tubes.