Is this a big deal? I kind of think it's different to the Facebook study because the aim (at least the publicly stated one) here is to improve their service rather than manipulate emotions like FB. Who seem to be so bored they're trying to be deliberately Orwellian.
Also I like the OkTrends blog so it's good that it looks like it's back and that the guy who writes it has a book out.
I guess it's probably all about consent but that leads to things like the EU cookie ruling that ends up just being annoying.