Facebook users without these users knowing

In 2012, Facebook conducted an experiment on 700,000 Facebook users without these users knowing about the research. Facebook divided these users into two groups and displayed more negative content in the news feed of one group and more positive content in the news feed of the other group. Facebook found that those users who had more negative content in their news feed were more likely to make more negative posts themselves. The opposite was found with the other group as more positive posts were made by individuals who experienced more positive content. You can read more about the above study here.
https://www.wired.com/2014/06/everything-you-need-to-know-about-facebooks-manipulative-experiment/
Considering what you have learned about ethics in research this week and throughout the course, what ethical issues are raised by this Facebook study? What consent issues were present in this Facebook study? Could there have been any issues with regards to vulnerable populations in this study? How would you feel if you found out that you were unknowingly part of this Facebook study? Finally, considering the hypothesis of the study that more negative content would lead to more negative posts and vice versa, could Facebook have addressed this hypothesis without manipulating the news feed content of these 700,000 Facebook users?