If Facebook's new contextual advertising scheme is not a blind scheme - i.e. marketers can consult with Facebook on what behaviour to market to, but Facebook does all matching of individuals to behaviours internally - then it really should not exist. If it is a blind scheme, the alarmist response to the announcement is probably overblown. It would just be user centered versions of what amazon is already doing. It's interesting that this is causing such concern where Amazon didn't even though it's clear that if you like books Amazon has had a good idea about exactly who you are for some time now - and thorougly marketed to you. They know where you live too.
The danger of course is in the practice that Facebook has not been able to curb wrt. the apps feature: Commercial marketing as cold reading based in information you aren't expecting marketers to have about you: "Friend X wrote something on your Y wall" - only you don't have a Y wall, you actually need to purchase that first.