Facebook Is Not Spying on WhatsApp. Is Not
It hit me that the pressure on Facebook to stop this from happening is paradoxically forcing them to actually spy on people in a way they currently are not. What I mean by this is that in order to rule out any recommendations based on a coincidence or double reveal, they actually would YES have to spy on their users. Yes spy on users, in order to avoid the supposed creepiness. So that then they can see, oh this user claimed on Whatsapp he enjoys eating cat food (this is my go-to example for Whatsapp alleged creepiness), let's make sure not to show any ads for cat food so he doesn't freak out. And for those purposes double reveals and coincidences are identical, they both require mining the Whatsapp messages to identify them because they both appear on Whatsapp messages.
Forget completely about the impact on ads of showing users ads about anything except what they want, which is a real concern for Facebook. It's a for-profit company after all, it is meant to care about that.
But no matter, focus only on how public pressure affects them: the public is basically forcing them to do precisely the misdeed they are not doing, that they are accused of doing, in order to appease its accusers.
Food for thought.