Many people do not realize that they are participating in new thing called "cyber bullying". Most teenagers just get into an argument with an enemy/foe and then insult the person. This is NOT cyber bullying. Cyber bullying in my eye means that the bully preys on one person, and continually does it until they force a reaction. 

But, for the teenagers who actually do cyber-bully, I believe they do this because face to face confrontation is always harder and not as humiliating. Also there is an element of anonymity on the internet which results in a delusion that there is no way that they will be found out. If I witness an innocent person getting bullied, I probably would not physically get involved (It is the 21st century, people are crazy!). But, I would definitely try to talk some common sense into the person.