No, I mean he literally is a fictional character. He was invented as a character for a scary story that some guy wrote and posted on the internet.
Minus the "internet" part, the above probably pretty accurately describes the creation of every Pagan deity in every heathen pantheon under the sun. All that separates the creation of Slenderman from the creation of Crom Cruach, or Pazuzu, or Baal is the medium of transmission and a few millennia. The intention is essentially the same.
However, I guess it boils down to the fact that, if you do something with the intention of summoning demons, even if the ritual is just something some stupid kids made up, you run the risk of it actually attracting the attention of demons.
I think your assessment here is correct. Man's fallen nature is today just what it was 100, 1000, or 3000 years ago. The inclination to idolatry is inevitable in persons - and cultures - which live apart from Almighty God's grace. And the demons certainly won't pass up opportunities to receive idolatrous adoration and lead souls into hell simply because they come in such prosaically modern forms as store-bought Ouija Boards and Tarot Cards or a "viral" internet "meme."