One thing I've noted is that female friends who I felt had a bit of a crush on me in the past delete me out of no where when they start dating a guy. Of all the friends on their lists, I'd get deleted when I didn't even do anything to them. This makes me feel like the girl gave it away that she once had a thing for me but has now moved on permanently after sitting on the fence for a while. Agree?
It takes a whole lot for me to go and delete someone off Facebook. They'd have to be posting things that I get offended by, constantly, and even then, I'm usually too lazy to bother going to their profile and deleting them.