[nness]

That's not quite right; Even in circumstances where participants are mislead for other purposes, they are still informed that they are taking part in an experiment.

The issue here is that Facebook conducted behavioural experiments on participants whom were not informed that they were part of a study. It is unethical. Whilst the outcomes are tame for those involved, the shear number of those involved and Facebook's influence and presence in everyday life makes it all the more alarming that they attempted it in the first place.