All Facebook users have agreed to be part of research experiments. It's in their ToS. If you got people to agree to allow you to enter their homes for research it wouldn't be unethical for you to do so.
ToS are a copout, and in my eyes a tragedy of the modern legislature around software services.
Every single company in the world knows very well that 99.99% of their userbase won't read the ToS, and use this to do whatever they want with their users' information and privacy.
There needs to be dramatic improvements in that area.
Oh lord, not this "break into the house" fallacy again.
Your FB profile is not your house; it is just some data you have shared with FB. FB decides what to do with the data: how to share it, where to share it, when to share it, who to share it with, etc.
Everybody knows that FB _already_ manipulates the feed to change your mood: to make you more engaged with the site; to make you click more on ads; etc. It's been doing this basically for ever.
Would it be unethical if I broke into your house to randomly place pieces of candy and $5 bills in your drawers?