Does anyone have evidence to back this up? I didn't look too hard, but the Wikipedia citation links to a page which links to pages that don't have any actual information on them.
I also searched the publishing site and found nada. Of course, I didn't do an exhaustive search, but still...
Just as an aside, I wouldn't be surprised if some college grads never read a "proper book" after college. I don't think that means the end of literacy - we do read a lot on the internet...
However, if you are counting only non-fiction then the Amazon data seems to confirm.
True fact: There are 4.25 million babies born each year in the U.S., and the most popular parenting books on Amazon only sell around 75 copies per day. (I haven't looked at the data in a couple years, but you can get it via ranktracer or similar.)