Of course you have to take in consideration pretty much everything when judging a movie, and first thing that comes to mind is the budget, but even with a micro one, The Slender Man is very bad.
So, the plot, clearly could have been written by a 10 years old who saw some Freddy and Jason movies, it is so weak you won't believe it as it plays along. Characters were cardboard tops, no scares whatsoever, extremely little views of the Slender, very dumb decisions taken throughout the entire movie and one ending that will make you weep.
I had my share of bad found footage, but this one is top 10 worst ones clearly. If the project itself was a hat too big, or it required a decent budget, then don't do this. Don't ruin it for others just so you can put your name on a movie called The Slender Man! It was horrible!
Boring, bad, predictable and sad. Sorry to say so, but this one right here is an bad example of found footage. Cheers!