Juice Analytics does great data visualization work.
Here's a link to their interesting graphic: http://internetmap.juiceanalytics.com/
Here's snapshot (apologies to Juice for my scratching red ink over their nice design):
Notice that at the bottom of the graph is a checkbox to toggle on or off the "Show All Others".
Toggled off, the graph shows the share of visits by site, as per ComScore, just considering the leading sites in each category. Toggled on, the graph shows the share of visits by site, including all sites in the category.
Checking that box includes the Long Tail of site traffic.
And when the Long Tail is included, the Long Tail wins. In (almost) every category, the dominant category leaders have effectively no share, in absolute terms.
In almost every category.
The head wins in Search, Portals, and Auctions. In those three arenas, the category leaders own the traffic, and the smaller sites get very little.