http://www.forbes.com/sites/kashmirhill/2012/02/16/how-targe...
More seriously, what you say may be true, but that's a stretched definition of "purchasing intent". Google wins here not because they happen to get eyeballs of surfers interested in diapers, but because people actually use Google to find places to buy diapers. That's a tough nut to crack, and the point of the linked article. More generally: facebook has "eyeballs" and page views, but search providers have the people with their wallets already out.
Facebook has way more information about me than LivingSocial, so I assume they could do a better job of that.
Interesting post on Target.
Honestly my intuition says that there isn't that much money in that kind of "buy this, you don't even know you need it" advertising. Consumers generally know what they want already.
BTW, Amazon's revenue dwarfs Facebook's but Facebook made more profit last year than Amazon ($1 billion vs. $900 million).
And yet there are plenty of ads there. Why? How do they work?
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If Facebook knows that you recently got married, and that you just surfed over to a baby website (equipped with the "Like" button), you might be intending to have a child, even if you're not yet pregnant. They can start selling you a daily-deal on diapers before a kid exists to soil them.