Obligatory Sold a Story podcast link.
I can't help but feel that a lot of this is deliberate, the end result of decades of dismantling the public education system to further divide kids into the upper class in private schools, religious fundamentalists in home schooling, and everyone else abandoned to keep the population uneducated and in worse economic precarity.
Somebody please tell me that the kids are alright 
i think the caveat gotta be comprehension of some degree. the people making and popularizing novel communications, and by necessity using it a lot---they're very literate, knowing rules and bending them. but you can just learn distortions without really knowing why or what the differences are. you can pick up the context & appropriate uses of a contraction or acronym without knowing what they actually mean---and there's many old words that are just calcified versions of that which nobody but pedantic nerds know the origins of.
nobody's clever for using OK now, but it would've been a sign of literacy (and iirc aristocracy) back in the day
Certainly possible. But I don't think social media is degrading that comprehension any more than the newspapers degraded it a century ago.
Just like any language skill, its a learned affectation. You're still more literate for knowing it than not.