I don't think it's ranked b/c it depends on who your POI is. So you may have been the first choice of your POI and if another applicant declines, yours may be the next in line to choose a student. However, you could also be the second or third choice in which case it would be much less likely you would be admitted. People definitely get off the waitlist though. You could email the grad admission coordinator and ask how big the waitlist is. I'm sure they get that question all the time.