I actually have it from Fark, not sure what year it started being used there, but I have been an active member there since just before 9/11 and stayed because of the awesome community and responsiveness to the events during that time.
Portuguese speakers have long used "isto" (translated as this) as an affirmative statement, regular usage of a term meaning "this" as an affirmative didn't start with the internet.
I remember seeing it on somethingawful as far back as 2003. They started a lot of memes, tl;dr, o rly? and so on.