Three cutoffs are established, 46, 71 and 76. Students scoring above 76 are encouraged to take 8.012, students below 46, 8.01L and the rest, 8.01. Students scoring 71 or better, but less than 76, have the option to take 8.012.
This article from The Tech last year states that 865 freshmen took the exam and 430 were exempt because they scored 5's on the AP Physics exams mentioned above. The enrollment figures are...
8.01L - 109 Students 8.01 - 600 Students 8.012 - 86 Students
8.012 is limited in size, so I am not sure if these numbers are indicative of the passing rates on the diagnostic exam. But 2011 was the first year that there wasn't a waiting list to get in. Also, in older (1990's) articles The Tech reported passing rates from 50% to 75% (there was fluctuation from year to year). In 1995 the average score on the diagnostic exam was 78.
It amazes me that MIT freshmen would have so much difficulty with this exam.