This is my page concerning the us.* usenet hierarchy, which is for discussions specific to the United States.
Why us.*? Well, I'm sure various people have various opinions/motivations, but to me it addresses the need for specialized communities in usenet as it grows. The Big 8 has sometimes been able to specialize according to topic, but some subjects are not well suited to such division. Of course, it is also aesthetically necessary for there to be a us.* now that the Big 8 is no longer US-specific (and/or "should" no longer be US-specific).
The official us.* page is usenetnews.us and it has official documents and such.
Trying to summarize the traffic in the hierarchy is likely to make this page dated but do look around. See if anyone is still posting limericks to us.arts. See if we still have Real Live Accountants(TM) in us.taxes and Real Live Lawyers(TM) in us.legal. See if the flames are cross-posted or indigenous.
Disclaimer: this page is not run by the us.* hierarchy maintainer and random comments are mine alone.