For the particular Facebook instance, web sites do that kind of testing all day. I don't see an issue.
If websites do things like this all day but society as a whole believes that to be immoral, it's going to be done in the dark and the resulting knowledge doesn't go into the public domain. A lot of value that society could have doesn't materialize.
Is the harm that the average ethical review board prevents less than the harm that they cause by preventing research from happening? Are principles such as requiring informed consent from all research participants justifiable from an utilitarian perspective?