Here’s my theory: People are reading less and spending more time watching TV/Movies.
Sure, this may help to expand vocabulary, but does very little for spelling or grammar. When I was a kid, I used to read voraciously. As a result, I tend to notice spelling and grammatical errors without trying.
Most of the reading kids are doing these days seems to be on-line. If they are reading Wikipedia or other such things, then the quality is fairly high, and it would seem to be a benefit. However, prowling Facebook, YouTube, MySpace, Twitter, and other dross merely exposes them to other people’s sloppy writing.