I agree fully that such conversations over forums have a far different effect. However that doesn't change that when a person reads a post, they discern their own message from it. Just as when you post you might gear your writing to deliver a message in such a way that it has greater influence than had you simply stated it.
Many people read something and hear it in their own head and accept the idea as their own. Or perhaps they create connections that might not exist, saying to themselves that I already thought of this and I agree. Or haha as many have experienced refusing to understand the message and simply placeing ignorant reply's of this idea/concept/message sucks, without a full explanation.
There are advantages to a conversation with a person face to face, that might go beyond that of a forum as well. (Although the ability to state all things without being interrupted and without them being misunderstood on a literary level are rather large)
In a face to face conversation for example many people will begin saying something and then be unable to complete what it was they were saying. You get their half formed thoughts. Perhaps they even change what they originally thought and correct themselves and start over. Another major advantage of the face to face conversation is that people can communicate alot with body language. If I did a speech in front of an audience, I would say only about 20% of my effect is due to the content of what im saying. The other 80% could be broken down to them likeing me, them understanding what im saying through actions, or visuals, and of course them understanding how important or less important something is through my interest, how loud my voice is, and how I emphasize certain words.
Now this can almost all be done in text. I mean I can say something in italics , I can underline important facts. Hell I can even bold or change font sizes.
Capitals, spacing etc, all of these things can have a tremendous effect on the way a message is interpreted, but the question is, can it have the same strength as the physical, vocal, and visual characteristics of a face to face conversation.
(And although I always should, I never go back and edit my post before submitting, it might make it clearer, but it might take away something that I hadn't noticed or realized as being important, that someone else might see.)