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.