I learned this info by using a 4chan archive tool called yuki.la., and came of my own research.
It started with an off-topic /mu/ sometime around 2009.
The thread was titled Syd
The op used an image of an old man cringing with the comment :
"Your face when you first saw 4chan"
Other anons showed what their faces looked like when they first found 4chan in the image they posted, then jannies took the thread down. Before, around the summer of 2007 couse avatarfagging was started/became popular by newfags and both was hated by oldfags.
But oldfags slightly warmed up to both ideas by making joke/troll threads. Well needless to say, during raids newfags would say shitty 4chan memes leaving a ton of digital footprints to 4chan. What was worse, is that 4channers would (even to this day) act like an imageboard user outside of 4chan
On youtube videos, they would leave comments like ">mfw 30 seconds in the video"
Other normalfags would be confused by this and ask the 4channer what > or mfw means and the 4channer would get flustered by this. This would not just be limited to just youtube though.
4channers would make facebook groups and use images as reactions and normalfags picked it up around 2012 and called it reaction images, which later evolved to the normalfag "Relatable posts"
For instance : "When you drop all the chips" (and then the normalfag shows an image of dark vader in anguish when he learned that padme died after giving birth.)
1 2007 avatarfagging
2 2009 tfw/mfwPost too long. Click here to view the full text.