The research firm explained that one way of understanding a site's ability to monetise its content is the number of display ads it serves.
ComScore said that nearly half of all display ads seen by US internet users originate on just five properties, illustrating the concentration of advertising among a few sites.
Display ads are defined as any rich media or static image served and loaded into a web browser, but do not include video streams, text or paid search ad impressions.
Alistair Sutcliffe, vice president of comScore Advertising Solutions, said: "Just a handful of publisher properties deliver a preponderance of display ads.
"When one goes beyond the top properties it is clear how fragmented the online advertising market is, underscoring the importance of advertising networks in aggregating the long tail."
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