I have some trouble with Mr. Robb's logic, but I do feel that he makes a point.
Blogging, and it's retarded siblings twitter and facebook are, at their core, vanities. The create spaces where individuals can have a small audience. These are the dedicated core users that Mr. Robb speaks of.
But to say that these users create the value might be a bit of a stretch. In a sad way, the social networks are the emotional equivalent of beauty contests and high school student body elections. It is an arena where an individuals can carve out a readership and a following for their vanity. I am not at all certain that it creates worth.
The Trap Mr. Robb falls into is the one embodied in a crude viewing of labor theory of value, where the labor that someone puts into something defines it's value. There is no discussion of the skill of the labor, there is no discussion of the quality of the output.
I don't think that social networking is anything other than a fad. It will go the way of cruising and disco, a monument, carried on by some odd enthusiasts, imbedded an a culture that has moved beyond it.