'Flip-flops' in 1978?

In the first scene, Johnny and the guys are talking about knocking a bottle off of a car fender with a soccer ball from 20 yards. Gracie says she can't do it because of her footwear, which she calls "flip-flops." That's the current term for what she was wearing, but I don't remember hearing it in the USA in 1978 (I'm a couple of years older than Elisabeth Shue). I remember them being called "thongs."