AdAge has the details: Last year, social nets, largely represented by Facebook and MySpace (NSDQ: NWS), took in an average CPM of just $0.56—versus the $2.43 CPM for web ads in general. Therefore, comScore concludes, if it weren’t for Facebook and MySpace, CPMs would be roughly $2.99. In other words, social nets pulled CPMs down by as much as 18 percent over the past 12 months.

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During Q1, comScore noted that Facebook served 176.3 billion display ads on its site, or 16.2 percent of the total. The social net’s numbers were even more impressive when compared with Yahoo (NSDQ: YHOO), which served 131.6 billion banner ads to across Yahoo’s portal, and Microsoft (NSDQ: MSFT), which delivered 60.2 billion display impressions during that period. It’s worth noting that comScore doesn’t count ads that Yahoo distributed across syndicated sites, which contributes a great deal of its display revenues.

That’s one of the reasons that publishers like Yahoo still make more money than social nets like Facebook. But the glut of inventory that has depressed display prices through the depths of the recession is continuing to hold back the space despite this past year’s tentative recovery.

But there’s still hope for web publishers looking to blunt Facebook’s impact on CPMs. The social net doesn’t want to challenge major publishers directly on premium display and has in fact avoided deploying any of the larger display ads that have captured sites’ attention since the Interactive Advertising Bureau and the Online Publishers Association began promoting new standard formats last year. Publishers insist that offering a larger, uniform display format for advertisers will inspire greater creativity.

The thinking goes that in turn, more creativity will spur greater ad spend. That may actually be happening right now. But overall, it will have a limited affect against the never-ending flow of inventory from social nets and the laws of supply and demand that sites like Facebook will continue to exert on advertisers’ finite ad budgets.