With Google you bid contextually, that is, you are guessing that because the page is about (for example) "The Beatles" that the people there are interested in The Beatles. This is generally right. But, you have no idea about the demographics of the viewers. Just knowing if someone is under 18 or over 21 pretty much means whether or not they have any ability to purchase your product after clicking your ad. And these demographic/behavioral trends go much deeper.
On Facebook you buy an ad based on the users demographics (age, gender, etc.) and what they explicitly said they were interested in. Most people don't list every single interest. Facebook, by pulling in all this external data now has a shitload more stuff advertisers can target ads on. Instead of targeting the tiny fraction of people who put "The Beatles" as their interests, now advertisers will be able to target users that are likely interested in The Beatles because they do things like, listen to The Beatles every week, or all their friends listened to The Beatles this month.
Its very likely that Facebook will be able to beat Google's CPMs with this deep targeting ability. Whether or not its planned, I expect that Facebook will attempt to syndicate their advertising and compete directly with Google
Users should be very concerned. Advertisers can figure out a lot about you based on you clicking an ad that they targeted to a narrow set of specifications. You might think the ad is about free ipods but its really only being displayed to users that did a certain set of criteria. This can and will be abused. To a limited extent it already is.
Back when Facebook first rolled out their ad network, there was an advertiser or two who figured out you could target females that were engaged, and by insulting their weight get them to buy their weight loss pills.
As you can imagine, Google needs a large Google + user base so they can build their own dat aset to sell ads based off of.
Ironically the next Facebook killer social network will likely have the main selling point of privacy. I'd sign up. Google + is not it.