My wife had a very early ruptured ectopic pregnancy (which she wasn't aware of), which nearly killed her. On the week that the baby would have been born, Enfamil launched an acquisition campaign targeted at expectant mothers that month, kicked off by a Fedex package containing bottled/powered formula (worth about $60) and including Facebook campaign targeting new mothers.
This was very upsetting to us, so I dug in, found out from the company where they obtained out information, and obtained the list. I spoke to a couple of physician and nurse friends who are pretty confident that it was a combination of the ER admission, an imaging claim, and a drug that was prescribed and filled in the hospital that made it highly likely to an observer that she was pregnant -- but didn't provide enough detail to determine that she left the hospital not pregnant.
It turns out that information regarding hospital admission, insurance claims and prescription fulfillment is available and slurped up by various parties, who are able to de-anonymize it. It contained alot of information, including accurate salary history, cars we owned, approximate credit score, where we bought baby stuff for our previous child, etc.
I make it a point to share this whenever appropriate, as it was a very painful violation of our privacy, and really threw me for a loop, as I couldn't fathom that our medical providers were selling our data to help pitch diapers and formula. Facebook and many other companies either has access to this information by buying lists, or can extrapolate by looking at advertiser patterns.