The online ad market may not be having its greatest year, but things should pick up eventually and, when they do, it's going to be a very different ball game than even a year ago. Already, display ads on social networks make up more than 21 percent of the display advertising market in the US, and the trend is increasing. MySpace and Facebook head the pack with a significant lead, of course, the two of them put together making up 80 percent of the display ads on social networks, according to numbers from comScore.

“Over the past few years, social networking has become one of the most popular online activities, accounting for a significant portion of the time Internet users spend online and the pages they consume,” Jeff Hackett, comScore senior VP, said.

“Social networking sites now account for one out of every five ads people view online. Because the top social media sites can deliver high reach and frequency against target segments at a low cost, it appears that some advertisers are eager to use social networking sites as a new advertising delivery vehicle.”

Of the 326 billion display ad impressions in June in the US, 68 billion were on social networks. MySpace had the biggest share with 9.2 percent of the overall market, followed closely by Facebook with 8.2 percent, even though it had three million more visitors that month.

This, of course, confirms the empirical observation that MySpace tends to have more ads than Facebook. However, things aren't looking so great for the rest of social networks as the remaining eight from the top ten add up to just 1.4 percent of the display advertising market.

But, despite the big share of the market, social networks aren't actually making that much money from display ads. This is caused by a combination of things: on the one hand, impressions on social networks are extremely cheap thanks to the huge number of page views they generate and, on the other, the online ad market is also affected by the economic downturn.