Websites are conducting research constantly. When Google moves a letter a pixel in their logo it isn't for shits and giggles; it's because testing showed them that it changed something for their benefit. That testing was an experiment and involved users who will have had no idea that an experiment is going on. When a company tries putting something new in high visibility spots in some of its stores, then uses that data to decide how to place products they are performing an experiment on people who haven't given 'informed consent'.
Literally millions of experiments like this are happening all the time. It isn't viable to inform users of all of them, especially as many people performing them may not even realise they are doing it and it isn't beneficial to stop them all. Read what Facebook actually did, it may be that it crosses a line for you even though it didn't for me, and that's fine; so let's try and come up with a line we can all accept rather than a kneejerk reaction of claiming that any experiment is bad, no matter how trivial, without informed consent.