FBX has been around for a while; yesterday was simply the first day that the demand-side platforms who are allowed to buy ads on it were allowed to talk about it publicly.
The new logo keeps the blue and gray trade dress that's familiar from Facebook's other branding. But — controversially, perhaps? — it doesn't include the social network's iconic lower-case "f" marque.
We've asked Facebook who designed it and when they get back to us we'll let you know.