The problem is not that they attempted to create an emotional response or manipulate people's emotions. As people are constantly pointing out advertisers so that all the time. People don't seem to grasp that there is a large difference between this and advertising.
The problem is the way it was done. People use facebook with the expectation that they are seeing a (reasonably) objective representation of what their friends are trying to express or convey. Facebook is the equivalent of the telephone in a telephone call. If the telephone somehow manipulated what you heard to make your friend sound more negative or positive without changing their core meaning that would be unethical without informed consent, just as this is.
A more extreme version would be facebook subtly modifying the content of what your friends post as it appears to you without anyone knowing it was doing this. That would be even more unethical. The problem is mirepresentation, the method by which they attempt to manipulate emotions.