There's a huge difference between A/B testing, designed to optimize your website with the direct intent to improve sales, and performing experiments on how different news feeds affect your users' moods. A/B testing typically comprises changes in button size and color, website layout, font variations, etc; should we lead with the price, or with the benefits, or with something else? On the other hand, what FB and OKC are doing - admitting to, and proudly! - amounts to wholesale experimentation on their users, with undisclosed intent - perhaps to make the users come back more frequently for another hit.
This seems akin to me to cigarette companies manipulating the nicotine content of their products. That didn't go over well when it was finally disclosed.
You can't just tell people you "might" experiment with them, they have to know and understand that they are part of an experiment. They don't have to understand the goal, they just need to know what they are part of, and they have to consent to that experimentation. One could argue that A/B testing should submit to the same level of scrutiny as other psychological experiments, but I think people generally understand and accept corporations' profit incentive. We don't accept the idea that a company might wish to screw around with our mood or set us up on a date when they know it won't work out.