I love that this story uses Slenderman's origin as a creepypasta, and expands on that in different ways. Creepypasta horror (or internet horror? not sure if there is a real term for this yet) is becoming one of my favorite horror genres. I really enjoyed the mixed media, and how this story leans into the different technology that let the original creepypasta spread. I also appreciated that he cryptid is kept relatively vague, the horror comes from Matt's mind more than the supernatural entity the book is named after. 

I love the commentary on how things can take on a life of their own - SM started as a story to win a contest, but has grown into an entire phenomena. And how that story can grow and branch out, and be interpreted in several different ways, and serve as inspiration for other stories. 

I really liked the ending. It's kept sort of vague, you don't get specific details; but there is enough there to feel like the story actually ended. The last section of texts broke my heart just a little bit, and I LOVE when a horror book can make me feel sad and scared.