This could become a problem:
In order to sign up for Facebook, users must click a box saying they agree to the Facebook Data Use Policy, ... there was no need to ask study “participants” for consent, as they’d already given it by agreeing to Facebook’s terms of service in the first place.
I'm no fan of "slippery slope" arguments in general, but if standard Terms of Service mean companies can begin to conduct psychological experiments outside of "do users like this interface or that one more?", we're getting into dicey territory. Something to watch...