To play devil's advocate: why does it matter? The purpose of language is communication, so it could be argued that if people feel they're able to understand and be understood sufficiently, classic ideas of literacy aren't really a useful metric. I suppose what modern language usage lacks in depth it makes up for in brevity and clarity, which seems like a natural correction to the low signal to noise ratio of a heavily interconnected world.

Edit - I don't mean to challenge the value of literacy so much as speculate whether falling test scores necessarily indicate the loss of that value. Language, how it's used, and how changes spread are all very different in the information age. How do we know traditional literacy tests accurately measure the role it plays in people's lives now?

As an example, written communication used to primarily take the form of writing letters. With instant communication, ability to put thoughts into words quickly has a higher relative impact on communication compared to being able to spend as much time as needed to find the perfect words to convey a message.