I would buy a new pair. If you want to try gluing it though try this stuff called Gorilla Glue. It works. The problem is that the nub that holds that strap down is broken. It may never take that well or just break again. Walking in flip flops that do not have the correct attachments is a b buster.
I am not sure if gluing that strap would work, I would either do one or two things 1) Exchange them for another pair if you still have a receipt. and maybe get a better pair. 2) Buy a new pair if they weren't that expensive.
Just buy new ones.
I have tried to repair flip flop breakages, and haven't been successful. We've tried glue and duct tape and nothing worked. These don't look like they were expensive, so get a new pair and move on! This time get a nicer pair with a heavier strap that might last you a longer time.
Had the strap just come out of the hole, superglue would solve the problem nicely, you would just put glue around the edge and then stick it back in the hole and let it dry. Since the strap in fact BROKE and part of it is still in the hole, super glue will not solve your issues. Get a new pair