No idea, but it was very early on while I was living in California and links were flying everywhere. I never did sign up though, and over the years I drifted in and out. After Ask Metafilter was introduced I drifted back in and stayed in. After excitedly emailing some people with answers to questions I finally realized it was past time to join.
Fun With Comics & Community
I put a webcomic my daughter and I created one summer up on Projects and was overwhelmed by the positive response when it was then posted to the front page. I loved sharing the comments with my daughter and showing her on a map where people were reading her comics.
It's an amazing community here. I have many good memories of links found but my first stop on the website is always MetaTalk to see how the site as community is doing that day.