Fair enough. I'd even go so far as to allow targeting based on information entered voluntarily on THAT site, such as if I give facebook my gender and age AND say explicitly that those 2 data points are fair to use for targeting - then they can use it (But that data can't leave the site - meaning they can't include adnet.com/ad?age=$age.
So, for example, if someone is doing a Google search for art museums in Los Angeles they could be targeted with ads related to art and ads related to Los Angeles. Interests in art and Los Angeles can be inferred from that Google visit, so would be fair game.
If the next day they Google search for refrigerators they could not be shown targeted ads related to art because no connection to art can be inferred from the current visit.