UberFacts says 56% of people say they fold their underwear, while 27% say they just toss it in the drawer. Not sure what the other 17% said, but apparently their answers weren't important enough to make the poll.
My underwear is tossed in the drawer, unfolded. There is no reason to fold underwear. Nobody sees it so it doesn't matter if it's wrinkled. Even if it was folded neatly, five minutes after you've sat in it (say, a really short commute in your car to work/school), it's wrinkled anyway. People who fold their underwear are wasting a fair portion of their lives for no reason. No reason at all.