Financial Times: "Google blamed the difficulty of making money from placing adverts on social networking sites for holding back its growth in the latest quarter, contributing to a 9 per cent slump in its shares in after-hours trading."
Techdirt has a small round-up of thoughts on the subject.
Content in different media is consumed in different ways. Ads are content. To work in a different medium, ad formats should reflect the medium's particularities. Facebook has made a step in the direction of offering a medium-specific format with Beacon. It might have not succeeded yet, but that doesn't make blanket display advertising on social networking sites any more effective.