I'm not convinced that social media causes a loss of reading skills. I suppose it is possible but I would want to see some studies on the topic. Anecdotally, I do find myself reading less than I used to. I took a number of English lit classes as electives purely for fun and enjoyed reading a number of fun works that I think would hopefully qualify me as reading above a 6th grade level. But that was many years ago. I haven't done a lot of reading in the last decade except for news articles about everything going to hell. Of the few books I have read, I read them for pleasure and each was lightweight, not too much analysis and explication required, one rather challenging history book about the lead up to the first world war notwithstanding, though it's difficulty is due more to more complex sentence structure and arcane vocabulary, and less to its erudite discussion of an already complex topic. Nevertheless, I don't believe I have had any difficulties demonstrating far beyond mere functional literacy you described despite my infrequent reading of anything longer than a news article or Reddit post. Still, this is anecdotal and so I would be interested to see if any scientific evidence exists to connect a loss of reading skills with disuse and to what degree those skills are diminished.