Facebook is a funny thing, with the way people act. Send someone an invitation to your leaving do and he responds by blocking you. Ho hum.
It is quite easy to find out who unfriends you. Few friends are "stand-alone" ones, in the sense that you have no friends in common. What you will find is that you idly go through a friend's friends list and suddenly you'll see Add Friend next to the name of someone who was your friend recently.
Normally, if I've found that someone has defriended me, then I send them a quick message to ask if things are OK. Two of the last three times, there has been an apologetic reply that they defriended me "by accident". Hmm, do you know how difficult it is to defriend someone? It's not like Twitter where you can unfollow by accidentally pressing on a button.
One question in any friendship is rift or drift. Have you just grown further apart due to circumstances? Or is there a specific reason? Sometimes it's embarrassment - you don't manage to catch up with one friend during university vacations, and so are too embarrassed to see them the next vacation as you don't want to explain that you were too busy to see them at the last one etc.
Sometimes you'll idly wander what someone is up to, see they're no longer in your friends list, and when you go to their page, realise you no longer have any friends in common. There are two reasons for this.
Like the lady who says that she has enjoyed her university time, but now wants to move forward so won't stay in touch with anyone from there, some people take the idea of never looking back to that stage. I remember one man I used to work with and then noticing we were no longer friends on Facebook, and then noticing he was no longer friends with anyone from his days working there.
Or people can look for temporary separation, which can be sensible. After my PhD it took me years to settle, and in part it was because I wasn't moving on and forming a new friendship group, relying on old friendships instead. It can be good after leaving somewhere to make clear you will avoid contact for a while until you have started to put down roots in your new place.
One thing to watch out for, if you see that a few people have defriended you in a short space of time, is whether they know each other - is it just a group of sporadic defriendings, or a pattern? Sadly, a group of defrienders who know each other could imply that there is a piece of gossip about you.