The Power of Network
For publishers, a network of ads means added weight in the marketplace to show advertisers and therefore extend the earning power of each site. For advertisers, it means a single buy will yield traffic from many sites extending their message efficiently and effectively.

"During 2010, as US ad budgets crack open just a little, look for an accelerated migration of ad dollars from traditional to digital media. Advertising on social networks will never attract a large share of marketers’ ad dollars, while the classic interruption/disruption model of advertising, whereby marketers insert unwanted, usually irrelevant ads as a price the consumer must pay to view desired content, will erode, if not fade away."

Source: Emarketer (12/31/2010)