Google had the same issues with AdSense in the early days but was able to tune the product to normalize value to an acceptable rate. For example, at the network level you can apply discounters per site depending on the ROI of ads on this site vs network-wide to reward the higher quality sites on the network. I'd guess that Facebook will make a move towards advertiser value at some point and find some way of discounting around performance, or allowing for differentials based on the demographic being targeted.
