The problem is, classically, lack of informed consent: the vast majority of Facebook users simply don't know they can be subjects of emotionally manipulative behavioral experiments at any time via UI tinkering, so it could be argued the TOS small-print waiver does not de facto ethically (nor legally) constitute informed consent proper.
An altogether different matter of course is whether you agree with the Belmont report principles in the first place, or the adequacy of principlism for social science research in general [1,2]. But as far as vanilla regulatory research ethics is concerned, this study should rightly raise several flags in any self-respecting IRB.
[1] http://www.uvu.edu/ethics/seac/Bulger-Principlism.pdf
[2] http://eprints.ncrm.ac.uk/85/1/MethodsReviewPaperNCRM-001.pd...