Another issue cropping up is eyeballs moving from desktops to mobile devices. The CPM for mobile is significantly lower than desktop which puts more financial constraints on a company that is only ad powered. These are real pressures affecting $FB and $GOOG right now. Based on KPCB (1) research, CPM for desktop is $3.50 and for mobile it's $0.75.

[1] - http://www.slideshare.net/kleinerperkins/kpcb-internet-trend...