More (not so) fun facts:
54% of American adults read below a 6th grade level.
21% read below a 5th grade level, which is considered functionally illiterate.
High immigration numbers don't fully explain it either, as first gen immigrants only make up about 1/3 of those with low literacy.
That's around when the tea party movement was taking off, so that tracks. You see the seeds of what we see today back even in 2008, too; but to bring it back to reading deficits, we've had problems long before then (which, obviously, snowball into what we see today, e.g. Florida banning sociology and such).