So Facebook is taking a lot of advertising budget to establish brands that used to be for print and television.
Simple example: Diapers. By the time you have a baby, Facebook knows that you as a couple are expecting and you will see ads establishing relevant brands. You would never see those ads on Google, because at that time you're not searching for them. This used to be the business of television advertising. So you know the brand Pampers long before you ever think of buying a pack of them.
[0] http://www.bloomberg.com/bw/articles/2014-03-03/advertisings...
Many (most?) people don't go posting anything about pregnancy before for ~3 months traditionally, and odds are you are going to be searching for symptoms when you first notice something is wrong health-wise.
When a doctor or test confirms you are pregnant, I'd be willing to bet that users are immediately pulling up their favorite search engine to figure out next steps vs. blabbing about it to their BFFs on FB.
Now, if you are basing it purely off of carpet bombing people targeting off of demographics, yes, FB probably has more solid data there as it is self-reported and not inferred. But you sure as heck won't get self-reported pregnancy info earlier than Google will know you are interested in it.
If anything, I'd argue Amazon would have that data in a similar timeframe if people start looking up pregnancy books.
This also entirely discounts the notion that there is 3rd party data sets at play which both Google and FB leverage to enrich their data.
All of this is not to say your point on the brand advertising is incorrect--I actually agree with that as the "feed" has become the new television. But definitely do not agree on the specific example cited.
Could easily lead to a situation like when Target outed a girl as pregnant to her father [1].
[1] http://www.forbes.com/sites/kashmirhill/2012/02/16/how-targe...
What's impressive is that WPP alone brings in around 5-6% of both internet giants revenue. I wonder how the few multinational advertising giants (WPP, Omnicom, Publicis etc) compare to a long tail of local agencies and individual advertisers.
[1] http://www.businessinsider.com/wpp-google-and-facebook-spend...
[1] http://www.inc.com/peter-roesler/google-vs-facebook-3-things...