That seems as a bit of a slippery slope. Also, I dislike FB as much as the next guy but I can't really imagine that someone would sign up for FB without realizing that something like this feed manipulation is a real possibility.
Most people on FB are using FB assuming it's a utility something like email, without realising that it is like email, if email had its own agenda. It's really easy to forget that most people in this world do not live/work in the echo chamber that we do. When I talk to them about the issues, sure, at best they might profess to understand or even be concerned at the time of that conversation, but by and large they won't change their behaviour and the conversation will be quickly forgotten.
Never underestimate the indifference with which people can view the inner workings of technology.
0. I see the fact that I find this hard to imagine as a form of "Expert Blindness" or "Knowledge Blindness". If someone has a better or more accepted term for this phenomenon, I'd like to hear it.
My problem is with the goal of the manipulation in this case. Its not something like "persuade people to view pages". Its (as I understand it) "make people sad" or "make people happy". As someone who personally battles with emotional episodes, could something like this have inadvertently triggered an episode for me? Can I prove it?
Some people have put forward the argument that advertising is by definition a mood influencer and we see it everywhere. However, that doesn't mean that we allow all advertising. We frown on tobacco advertising because it influences people to buy an unhealthy product. We frown on displaying unhealthy bodies because of the impact it has on our self-images.
tl;dr; I'm not saying feed manipulation is bad. I'm saying the goal of the manipulation in this case was potentially ill-conceived.