Wow. It is pretty crazy that they're all female. I wonder if they have an algorithm to figure out who's more attractive or if they just choose people of the opposite gender.
It's probably people with a lot of friends in your area. And if you already have a few friends, it's probably friends of those friends. Maybe pretty girls are just the ones with the most friends?
If so it would be a pretty simple algorithm - just pick the people whose photos get the most views