I grew up in Texas and came of age in the 1970s. The sandals were flip-flops, but sometimes I heard them called thongs. The butt-floss swimsuit didn’t exist as far as I was aware at that point. There were G-strings which were something you’d find only in a strip club, and those weren’t called thongs.
It was only much later when butt-floss swimsuits became common that I heard them referred to as thongs, and I thought it was a joking name because they shared a feature with the thong sandals.
I only heard of flip flops being called thongs in the last ten years or so. Maybe it was a regional thing.
Same. When I was a kid in the 70s thongs were flip-flops. After my teens it became underwear.
I think of the women’s underwear.
I don’t know if I’m obsessed with sexual thought or not; best guess is, I’m an average male. But if I thought about sycamore trees as much as I think about sex, I would definitely be obsessed with sycamore trees.
Look, if you show people a pair of flip-flops and ask what they’re called, more people will say “flip-flops” than anything else, at least here in the USA. But if you show people a pair of that distinctive women’s underwear most will call that a thong. We’re comparing the primary name of one thing with a secondary term for another. Besides, I didn’t think a G-string was exactly the same as a thong. Aren’t G-strings skimpier in front?
From the midwest this is my first time hearing that usage. I expected underwear or a thin leather strip.
I had never heard the term zories until this week. Maybe it’s a West Coast thing.
BTW: A renowned jungle explorer was exploring a jungle one day, when he was stricken by an illness, that, while it probably wouldn’t kill him, would probably make him regret that fact. His malaria medicine having proven ineffective, he made his way to the nearest jungle village and consulted the village witch doctor.
The witch doctor gave him a strip of some creature’s tanned skin, with seven knots tied in it at one-inch intervals, and instructed him, “Each morning when you rise, bite off a knot from the leather, and chew and swallow what you have bitten off. When you have eaten the entire thong, you will be cured!”
So the jungle explorer, having few alternatives, followed the witch doctor’s instructions. On the day after he ate the last bit of the leather strip, he was feeling no better, so he went to the witch doctor’s hut to complain.
“See here” he said indignantly,“The thong is ended, but the malady lingers on!”
Please don’t hurt me.
Growing up in Missouri in the late 70s and 80s, I heard the irredeemably tacky shoes called “thongs” pretty often. But i haven’t heard that usage in years. Now when I hear “thong”, I immediately think of the underwear/swimwear. However, as noted upthread, if I heard someone use the plural, I’d probably assume they meant the shoes.
Re: Zories vs. Thongs:
Grew up in NYC:
Thongs were flip-flops that had a strip of rubber that came up from the rubber base, went between two toes, and spread out to go back below the base on either side of your foot. Zories had a foam/rubber bottom, but nothing between your toes - just a wider strip of foam or rubber that covered your toes from side to side, with the front left open.
Boy, that took too many words to describe something this simple. 
I had a friend who spent a year in the US while in his teens.
One day at school, he stood up and asked if anybody could lend him a rubber.
Hell, I didn’t know the word meant that in the US, either. 
The first person I heard referring to thongs as “anal floss” was Elaine Boosler. Long time ago.