I think it's more a matter of scale. If Facebook are serving 260 Billion pageview a month then even if they were making $3.1B a year we'd only be talking about

I think it's more a matter of scale. If Facebook are serving 260 Billion pageview a month then even if they were making $3.1B a year we'd only be talking about $1 CPM. That is ridiculously low by industry standards, especially considering how well segmented they can supply an audience. As an advertiser buying on any other network I'd expect to be paying $20-30 CPM for the exposure so that's the scale of Facebook's underachievement.