Yes, here's a study on manipulating the feed of over 600,000 users, in order to elicit an emotional response. Without asking for permission of course, because apparently you agree to FB playing mind tricks on you when "agreeing" to their terms and conditions. Really, it can't get worse than this and they have no justification.
Here's the resulting paper: http://www.pnas.org/content/111/24/8788.full.pdf