This is a good post.

Pm is another way of communicating between each other for different reasons I guess.

One reason would be to keep the communication or subjet private.

Another would be so that not the whole world can't read it. (www)

Trust for sure is one reason.

Scare to share in a large group would be another reason.

Rejection.

Be judge.

Not understood.

Safer with one on one.

But then there is the other side of the story. By not sharing on main board, we will never learn to communicate in large group. Also, we will get less oportunities to have different views on a subjet. Never learn to deal with our issues(in a large group) such as trust, rejection, have a voice, say our oppinions, belong and so on.

The most important thing is that there should always be respect for each other in pm or main board.

btw For myself, I rarely pm and I don't post much either. So maybe my point of view doesn't count.