The most mind blowing fact I've learned so far in Roland Allen's *The Notebook: A History of Thinking on Paper* is that the term "commonplace" actually derives from the commonplace book (or better termed "common place book", since it was originally intended to be an organized and indexed list of material).
Apparently it became common to write or speak specifically to encourage people to put excerpts in their books, such that those sorts of passages became "commonplace". Basically early microblogging.
Greg Pak