Thanks, all! I've been offline and nearly forgot about this thread.
I definitely think the yarn is the big key. My first two pairs of socks were not made of specifically sock yarn, and clearly didn't have a chance at holding up! The one that just got a hole in it was sock yarn, and advertised to hold up well, "even in hiking boots!" I do wear them often, but still....
I have another couple pairs from different sock yarns that are going along quite well with no signs of holes.
Interestingly, mine don't ever get holes in the heel flap, but underneath my heel, where I walk. I don't usually walk around in them without shoes, but occasionally.
Anyway...thanks for all the advice. Seems like finding and sticking with good yarn is going to be worth the effort.