I'm beginning to wonder if Facebook has begun to realize that being the source of ads while temporarily beneficial to their bottom line is actually destroying their long-time value to advertisers. Chasing eyeballs for ads necessarily warps the things you present to those eyeballs and eventually you're in a race with every other eyeball chaser. The problem with that race is that you no longer have an undistorted understanding of viewer/user preferences but instead only understand their reaction to your attempts to capture attention and clicks, which is a more mechanical understanding of behavior rather than understanding underlying motives and preferences.
Ad companies have long specialized in the mechanistic side of things so Facebook really brings nothing new to table for them, and merely being "a giant audience" isn't especially worthwhile either since a "run of network" ad buy for Adsense or the like is cheap. Facebook's differentiator was "we have all this insight into users!" and while that was true at the time they began courting advertisers what I suspect advertisers and Facebook discovered was that those insights became less and less accurate as users responded to the changes Facebook began to make in order to try and increase ad effectiveness, thus beginning the distortion spiral.
I wonder if Facebook is making an attempt to return to the insight side of the business, allowing advertisers to use the info Facebook has regarding people as targeting for ads that occur away from Facebook. They return to a demographic/insight source rather than an advertising platform.