Using aggregate data (ie. not any individual's personal data) about interactions to study societal behaviour is unethical? On a website that you willingly sign up for, no doubt with disclaimers about how they're allowed to use your data?
As far as slippery slopes go, Facebook crossed that line quite a while ago...
When you publicly disclose the results, without any way for the data to be peer reviewed? Absolutely.
They're not slippery-sloping anything. They're saying your data is theirs, period.