What 4chan boards do you post on/lurk? / What mainstream Internet site did you never participate in?
I lurk /mu/; /tv/ and /lit/ from time to time. I hit up /x/ and /b/ every once in a while but their glory days are long gone. And I've kept away from /r9k/ for about two months now because it's poisonous. Even then, I still think it's fucked up my perception of females in a seriously negative, unrepairable way. I do think Rayne would get hooked on /r9k/ if he lurked it for a few hours though.
I never made a Tumblr account.