And mostly, getting the primary bully (there's always a primary, the others tend to follow his/her lead) away from his pack can really defuse the situation.
A good start, but does not always seem to solve the problem. I think it is a mistake to stop there - the others continue, and someone else could step into the role as ringleader, or the ringleader becomes a matyr.
And sometimes a knee to the groin, or an "accidental" elbow to the nose as you make a sudden turn, is enough to let them know you can't be bullied with impunity. That's a hard way to go, though. Most of the people the bullies like to pick on are the quiet, non-confrontational ones.
I understand your idea, but that means that you have to know who the ringleader is, and that you have to be sure you do not have the whole hoard of other bullies over you. And, as you say, the ones picked are often non-aggressive people.
I think the long term solution is to work with the attitude towards it, to make enough people realize how cowardly that kind of thing is. The problem is obviously on the rise, and something will have to be done. I find it vital that the net not be allowed to grow like a jungle with no ideas of right or wrong, and neither the rulers nor the police can really do anything about that. It is up to the users. An awareness that behavior on the net is not gratis or completely removed from any other life.
The WHY is basic in-group/out-group dynamics. The way they use the net is based on the perceived anonymity it provides. Exposing them and their actions for what they are has been helpful there, at least.
I think you are right: it is a combination of net and group behavior. You can then start asking why there are so many followers in groups, why the dynamic is the way it is. Maybe one answer is that the schools are more concerned with academic standards that with citizenship ones, but one is at least as important as the other.