i don't think its an ARG, cause an ARG has to reveal eventually what the plot line is, half of what makes this show so good is how mysterious and ambiguous it is, if they start unravelling it , it will lose a lot of its charm, also the team behind it are also doing short bonus snippets of contents in text form in a website (japanese only sadly) and a book, and when you put it all togheter it just sounds like a bunch of mostly unrelated surreal paranormal mysteries taking place in the same universe, to me this is much better and more interesting than an ARG.
some of the best episodes are the ones you have genuinely no idea wtf is going on ("final countdown" and "film inferno") for example