The problem of most ad based apps is that they trade attention for interest and purchase intent. The problem with facebook is that it's so good at getting your attention that you rarely visit facebook with the interest or intent of buying something.
The core difference of google and facebook is that when you go to facebook, you are engaged with your friends and are in a lean back mindset, while with google you are actively pursuing interest in something.
I think VCs should actually come up with standard KPIs around interest and intent than saying lame things like X number of users is worth $y amount. It would be much smarter to say things like, a user with a KPI of K is worth $Z this year, and $Q over a lifetime.
A Dropbox, and Apple user looks much different than a Facebook user.