One of the reasons the Slender-Man is so popular today, was Marble Hornets, a ground breaking, rule breaking, mind curdling Alternate Reality Experience, or as some people think it is (In what it is not) and Alternate Reality Game, (ARG).
Marble Hornets is directed by 3 fine chaps called Troy, Tim and Joseph, they all run a company called THAC (Troyhasacamera), and the stuff they produce is pretty funny.
Marble Hornets is in it's 3rd season, and started in 2009? (Correct if wrong), and is responsible for the common "Slenderman" logo (It's actually called Operator symbom) which is a O with an X through it. Marble Hornets caused a ripple in popularity with the Slender-Man, with many other ARE/ARG's following after it, Like EveryManHybrid (EMH) and TribeTwelve (TT) in fact, the controversial horror game Slender was primarily based on it.
Here are some acronyms to use if you want to make posts some what shorter.
TO (The Operator, who is the Slender-Man in this series)
Masky (Masked Proxy?, revealed to be Tim)
Hoody (Hooded Proxy? revealed to be Brian, but please correct me if wrong)
OS (Operator Symbol)
TJT (Troy, Joseph and Tim)
I empathised the word Proxy, due to the status being debateable from what the Fanon/*Canon?* for a proxy set by other ARE/ARGs has been set as, (Example: The Collective from TribeTwelve).
All of the entries are available on Youtube, via the channel called Marble Hornets , there is also ToTheArk, which has VERY important things relating, and messages to Jay (The main character, played by Troy), Tim (Second Main character, played by Tim) and Alex (2nd main Antagonist, played by Joseph). Alternatively, you can buy the DVDs online.