Slenderman and little internet searching
Ok, I looked at creepypasta and they mention SM is fake and say where he originated from, in parentheses they say "or is he". I found the original story post on SM from something awful dating from 2009. They all clearly state SM is and was photoshoped and on to congratulate the guy who gave SM a story/background. He was even approved upon within the origan SM story page. All users there clearly state he's fake.
So with youtube users posting additional comments, creepypasta and somethingawful all claiming it was a photoshop contest (dating back to 2009) I cannot for one second believe these girls didn't know it was all fake.