This data is extremely extremely valuable, so my guess is that they were sitting on it until they were sure this was the best business decision. Also they already have their own ads so they have certainly been using this data internally all along.
I just genuinely thought this had been the case for at least ten years. That's what makes it so surprising.
That they haven't monetized it like this when this is how basically everything is monetized and they spent so long burning money hand over fist just boggles my mind.
Marketing lead-generation and sales conversion is what every company does with their customer list eventually (sign of a failing business model.) Whether it is internal revenue generation, or a 3rd party... this is what most large web companies trade with Marketers.
I would be more surprised if they kept peoples privacy, as even your credit card company sells the purchase data. =3