[ubernostrum]

"knowledge of infringement" basically boils down to "you know damn well what you're running here". It's not that every site operator has to actively police every single submission for possible infringement; it's that if even a ten-second glance at your service reveals obvious massive presence of infringing content, you don't get to pull the "well, gosh, we never knew" and claim safe harbor (this is the "is not aware of facts or circumstances from which infringing activity is apparent" clause).