The original blog can be found here. Its author eventually went onto UnFiction forums under the username "Knolly", and had this to say about the ending:
This entire "reveal" was something I was actually fairly hesitant to do, simply because I obviously would be expanding slenderman lore beyond what is already known. However, the truth is you really can't have that interesting of a game going on for that long based on the established slenderman lore, so I decided to take the plunge. Some people might like it, some might hate it, but you can't win 'em all.
The second notebook was honestly not that full of riddles. Looking back, I have a feeling you all may have been overthinking it a bit. As I said, I believe that some of this was a failure on my part as a storyteller, but perhaps if you now know what it means it might help make sense of it all.
Have you ever wondered if everyone else actually thinks and feels things like you do? One of the great mysteries of life is that you actually have no way to prove that everyone else is as conscious as you simply because it's impossible to verify. So thus we come to the idea of Yggdrasil, the world tree, and its nine worlds.
The idea is that at any point in time, there are only nine truly conscious beings/people. Because most of the population is equivalent of a robot, eight of these conscious being live rather brilliant and wondrous, yet ultimately short lives. This is because the ninth being is Slenderman, who is ultimately responsible for killing these people and therefore "resetting" the worlds. He is the only way that these people can be killed (ie the car crash), and thus one day, everyone who is truly conscious will have to face him.
The "unknown" notebook pages were:
V - This is meant as a "wild card."
VII - The implication here, which I clearly failed at explaining, was that Matt was one of the nine, and thus when he died, a new person was born to replace him.
The point of the "wild card" is that I sent out a number of pages to people, but I obviously couldn't send one to everyone playing the game. Thus, you, the reader, obviously believe yourself to be a conscious being, so that means that the "V" page must be for you, right?
When slenderman is hunting one of the nine, he will not stop until he finishes them off. First with Matt, and then Logan. Logan mentioned how he knew Joshua would be next and apologized, since his death meant Joshua would immediately be in danger.
And thus you, the player, are also one of the nine, whether you received a page from Logan, or if you know that the "V" page belongs to you. So now, knowing that, I would like to repeat the last words written in the second journal:
ALWAYS TURN AROUND.
You just might be next.