I disagree. If facebook is testing content like this then they explicitly know which content makes people sad. To manipulate someone's feed to push them stuff that knowingly makes them sad for the purpose of experimenting with someone's emotions, that is highly unethical to me.

There is a massive difference in studying and directing traffic to help make traffic flow better. The goal there is to improve driving for everyone. Facebook's goal is to intentionally make people unhappy for the sole purpose of making more money.