You're right. Nobody want approximated, imitated, false, or empty data.
And you're wrong. Facebook data is not vague. People use Facebook to write about themselves. And to announce it to their friends. People don't lie to their friends. And that's the miracle of Facebook to marketers. People write truth on Facebook. Sheryl Sandberg knew it, so she didn't hesitate to get aboard.
Unlike Facebook, Google data is poor. They have only approximation and best-guess. People don't want to leave any personal stuff on Google. Google is a device for work, not a personal entertainment. They sometimes leave bogus data to avoid Google's tracking. Actually many people do. I had over 5 Gmail accounts for spam mailers and crappy web-service membership. That's the vague data. They can't even dream some relationship data between users.
I don't understand your story about 23yo woman. What marketers want to know is the only thing - how can I find a person would like to buy my product?. Everything else is just a clue and device to track them down. Age, sex, location, taste, marriage… All are just parameters. But Google has only that stripped down parameters. No more. Those parameters are better than nothing, but completely useless when compared to what Facebook offers.
Facebook offers: (1) whether the woman actually married or not (2) who's' husband (3) what's their jobs = income level (4) whether they have children and how old are the children (5) when did she moved recently. She don't need to write about her children. Because Facebook will analyze photos and will recognize children's age, race, sex, and current living location. In addition, Facebook also let you know how many friends she have, and how are their children… recursively.
If you're in a diaper company, which profile would you prefer? I won't even consider Google data. That's just a crap when compared to Facebook profiles.
Of course, those data won't be offered as is. That's illegal. But each companies know how to deliver those data to their customers. And obviously, Facebook is expected to have much more fine gained control on choosing advertising target. Marketers also know the difference of that methodology. "You can pin-point bombing on Facebook."