That's a super interesting question. I suspect it is because Facebook dominates the market for advertising-that-tracks. They're just not as good as other players at advertising-that-doesn't-track.

So when they lost inventory (e.g. for retargetting), that is a direct loss of revenue for them. The question is, what did the company with the budget previously spent on those targetted adverts do instead? Did they buy less well targetted adverts elsewhere? Or up spend on offline marketing? The economic question is, how effective was that - more or less effective?

Obviously losing 10b dollars is bad for Facebook. It isn't clear that it is worse overall for ad spend, or economically.