Here’s a great article by AdWeek’s Cathy Taylor on the value of engagement vs. raw page views and click-throughs:
Measured by statistics such as page views, the biggest social-networking sites certainly rival the longtime biggest sites on the Web. But a closer look at their traffic shows that, in fact, they differ markedly from their older competitors. Unique audiences are smaller, and the number of page views per person—especially among the youth demographic that accounts for most of the traffic on sites such as MySpace and Facebook—tends to be much higher. So, while social media rewrites rules of content creation and distribution, it is also poised to revise the analysis around which metrics are important when evaluating advertising properties, as well as what constitutes a successful ad unit.