~2010 Literacy rates in the US are roughly 15ish% fully literate (it's a bit tangled to try and reduce down to a single number; the documents didn't give one, and the data files are in some odd format), according to the US literacy groups and their surveys. It's also worth noting that literacy for the councils on literacy is a pretty low bar.
While I can't speak to total trend (measurements of literacy have changed, as have the prevalence of testing), it's fairly clear in a qualitative fashion that total reading capabilities have declined over the last 100 years. Examine pulps (cheap entertainment books) from the late 1800s, along with childrens' books of the time... significantly more complex paragraphs and much larger vocabulary.
I'm going to demand a cite for the claim that only about 15% of the U.S. is literate.
I'll also note that there's a distinction to be kept in mind between literacy (mastery of the artificial skill of using the written word) and fluency speaking one's native tongue (a skill all mentally healthy human beings pick up naturally).
First, http://nces.ed.gov/naal/ is the organization I was doing my reading with. Their surveys span the last 20 years. There's a 200-page PDF describing the surveys.
Second, "literacy" is, as you point out, a nuanced term. NAAL has broken it out into 3 categories with 4 rankings possible from Below Basic to Proficient. Based on my reading of the survey questions, I drew the line of "literate" as "proficient".
So, http://nces.ed.gov/naal/kf_demographics.asp is a summary page, and "
Percentage of adults in each prose, document, and quantitative literacy level: 1992 and 2003" is the graph.
You can see that the percentages are not collected into a single number. I made the ENTIRELY GROSS assumption that proficiency in one area probably leaks into the other areas. I would like to calculate the actual "Total proficiency" score based on the data though.
While I can't speak to total trend (measurements of literacy have changed, as have the prevalence of testing), it's fairly clear in a qualitative fashion that total reading capabilities have declined over the last 100 years. Examine pulps (cheap entertainment books) from the late 1800s, along with childrens' books of the time... significantly more complex paragraphs and much larger vocabulary.