I started on IMDb for the movie 'Alexander'. That was early in 2005, I don't recall what month. February maybe?
I was there very ferociously for a few months, then I had to go onto my other fandoms; the last Star Wars movie came out in May and I was on TF.net for the entire summer, then in the fall, I went to another board for another interest, then in November, I think it was, I saw a trailer for BBM, got all bent out of shape and hit IMBd running full throttle.
I still visit there. Went to the 'DaVinci Code' boards, but they're crazier than the BBM boards.