Users of Facebook see it as a neutral platform for communicating with people they know. Consumers of the things you listed know it's top-down messaging coming from people they don't know or necessarily trust.
So, there is a difference. It's still a complex question, though -- is filtering or prioritizing based on emotional sentiment really different from what they are already doing with inserting ads and such?
I see it this way: they did a study, so it's fair.
Were they to filter posts by emotional sentiment as a part of their normal operations, I'd find it unethical, or at least something I might not want. But I'm totally fine with them subjecting users (including myself) to random research studies, as those are temporary situations, and with Facebook's data sets, they can have great benefits for humanity.
Perhaps Facebook should provide an opt-in option to for user to be a subject of various sociological experiments at unspecified times. I'd happily select it.
So, there is a difference. It's still a complex question, though -- is filtering or prioritizing based on emotional sentiment really different from what they are already doing with inserting ads and such?