Friends don't let friends use Facebook. And yes, there's a different link under each word.

(TLDR? A massive, and massively unethical, experiment in controlling people's mood by manipulating their facebook feed. Was given a pass by the IRB because facebook does it all the time anyway, so publishing the data was a net good. If anyone in the negative mood group committed suicide during or soon after the trial, there's a case to be made for corporate manslaughter, but of course no one will ever be allowed to know whether that happened.)