Facebook groups and "members in common"
I was taken aback at the "members in common" feature on Facebook when you are part of an eg. Expat group.
My friends list is private (because there are many hookers in there), but Facebook makes it easy to see which guys who have same hookers as friends, so presumably others can also see that I have hookers as friends.
Border agents sometimes look at social media and it makes me uncomfortable to realise how easy it would for them to link me to others in the mongering community, especially as not all are law-abiding.