Have you ever been unfriended on Facebook? I have. It happens to anyone at some point in their life. I don't recall unfriending anyone, for me it is very serious.
Some people do not realize consequences of unfriending, other people do not understand potential reasons that someone unfriends us. There are many many reasons.
Social network sites such as facebook raise visibility - it is clear who is our friend and who is not our friend. Unfriending someone more or less means we do not want to hear from that person or we don't care or both. It is almost like blocking someone, thus it has far reaching consequences. Your friends are the people you trust, obviously friends are not equal, some are closer some are remote. Considering that most people have at least a few remote friends on facebook unfriending someone is eliminating someone from a group of remote friends as well. It is potentially like saying, you are more remote than my most remote friends.
Unfriending does not mean there is something wrong with you, sometimes it can mean that the person who unfriends you needs to sort out their life. They may be going though a crisis and you are not helping them to resolve their problems. They unfriend you not to hurt you but to help themselves.
Unfriendsing someone can sometimes trigger very serious lack of self esteem, especially among teenagers. They may not understand why they have been unfriended and they loose respect for each themselves. A person unfriending it, may actually be aware of this. Emotional torture is a concept well known to some people as in a skilled manipulator emotional torture can lead to so called victim's complex and feeling of inferiority and submissiveness. Unfriending someone is like giving them feedback, they they do not deserve to be our friends. A person that has been unfriended may think... but why... what do I have to do to be your friend... etc.
Some people tend to send messages and ask why they have been removed. I don't do that. I especially don't do that when I have a suspicion that this person may be a control freak and also may want to practice emotional torture. In special situations, I may ask them but I would have to be sure they don't want to play any mind tricks with me. Unfriending someone puts you in a privileged position, it is like breaking up a relationship. No one wants to be dumped, people feel superior when they dump. Similar psychological mechanism.
For me being unfriended by someone is fairly serious, it is like being removed from a circle of friends that they trust. It means our destiny, whether we live or we die is irrelevant to that person. If means they don't care if we are sick or how we are feeling. You can imagine how much it can hurt, when you have loved that person. It is not "let's be friends", it is "I don't care what happens to you." Unfriending someone that have loved us or loves us is the most cruel and vicious thing we can do to them. Just don't if you care for human beings. If you want to do it for yourself, then think about the consequences of your action for another person before you press that button.
I have heard that people unfriend to ease their own pain, when they have loved someone very much, they cannot stand looking at that person anymore. I think it is ok to do that once you notify that person, otherwise it is again cruelty in disguise. Just my opinion, who cares anyway?