New York — Social networks like Facebook and MySpace account for more than one-fifth of all U.S. advertising traffic, but are bringing in the lowest rates for their ads, AdAge reported.

In fact, the publication reported that the networks may have dragged down the average online CPM (cost per 1,000 impressions) by as much as 18% over the past year. According to comScore, Facebook and MySpace have drawn an average CPM of 56 cents over the last year — compared to the $2.43 seen on non-social network sites.

“Social networks are going to be a challenge for everybody, as the sheer dominance of the impressions they’re making flood the marketplace with inventory,” Keith Lorizio, Microsoft’s head of U.S. sales, told AdAge.
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