As I tell my kids frequently, “NOTHING is free”. (Apparently Chris Anderson’s new book “FREE” is one exception.) Freemium and Freeconomics are very cool buzzwordy concepts, but at their core, the two concepts of “Free” and “Revenue Model” are mutually exclusive.
I think the real play is the segregation of which constituency is the “User” and which is the “Customer”. The “User” supplies the eyeballs, the traffic, and hopefully the Ad Clickthroughs. In this model, The “Customer” supplies the (Ad Placement) Revenue. In a true for-profit enterprise, there is no Free Lunch from the Customer’s perspective.
If you get something “for FREE”, chances are somebody’s paying. Chances are that somebody may be you, directly or indirectly. Just make sure you’re comfortable with that.