[Death by Mirrors]

I used to love how you could enter your interests on your profile, then you could click them and found other users with the same interest.
(back in the old days when this was a novelty invention. I also really enjoyed Robot Co-op's 43things, even though I believe I barely did any of the things on my goal list for real, but you just can't beat that moment when you discover there's 52 other people who also want to challenge someone to a duel or jump into a pool with their clothes on)