The girls' plan to infect us and with this direct Slendy's attention away from themselves and their loved ones in season one has some serious flaws (as for the second season it doesn't even work):
They never saw more than one Slendy at once. This doesn't mean there is only one.
Or that he/it can't be in several places at the same time.
Or bend the world so that several of his targets are where he is in the same time.
Or to terrorize them in 'out of time', giving him unlimited opportunity even if he/it is just one.
The Fourth Wall Will Not Protect You might be subverted. It's never established in their continuity that the Slender Man can only stalk those, who know about him. Quite the opposite: His going after one of the heroines' little sister (who wasn't shown to be infected) implies, that:
Maybe only in-person "infecting" works.
He can stalk whoever he wants.
They also fail taking into account that Slendy might likes to finish what he began before going after new people.
And even if they were perfectly right: People who watch Slender videos tend to be infected already. So he theoretically could terrorize us, but prefers them for some reason.
