Nonetheless, the AAUP document seems to show that even at US public doctoral and research universities, in 2005 non-tenure-track employees made up about a quart

Nonetheless, the AAUP document seems to show that even at US public doctoral and research universities, in 2005 non-tenure-track employees made up about a quarter of the full-time faculty and about 40% of all faculty if you include the part-timers (p. 18). There's also the issue of graduate students who teach or do research work. The numbers apparently included teaching grad. students, but only those which had been self-reported by their institutions as "employees" rather than recipients of valuable teaching/research experience (p. 10): it's not hard to imagine some under-reporting there.