December 17, 2008, I think not long after the site was first introduced.
1st October 2009, although I did lurk around the site a while before that.
Thank you so much!
It's on your profile page. Just click on "Read about me" below your profile picture. For you, it's December 5, 2008.
For me, it's October 1, 2010, and I haven't taken any long breaks (more than a week) from wiki editing. Things have been busy.