Calling this a fad isn't helpful. There's a reason why this is happening and if it's a fad, so be it, but it's one we all should understand and act upon.
What is the justification for believing this? What should we understand? I don't think anyone can really understand fads, its like why is one song so catchy and others not, quantifying that is very difficult. Why should facebook win out currently over the other social networks is an interesting study point,m but I fail to see why we should study facebook if that was your point.
