ASK HN: CVS Pharmacy Sharing Data with Facebook and/or Groupon?

Yesterday I had to buy a compression sleeve for my injured knee. I didn't do any searches on the internet, just went to the nearest CVS and bought one using my wife's CVS loyalty card. Today on her Facebook account she was presented with an ad from Groupon with a coupon for knee compression sleeves.

I never had problem with my knee before, I didn't do any searches for knee injuries or anything related to that before, and neither did my wife.

I was under impression that sharing of personal health data is prohibited, but it seems that if you buy stuff that's over the counter, there is nothing stopping companies from sharing it with 3rd parties.