I will add they did this experiment partially in response to an academic study that found viewing happier posts on Facebook made people sadder (explained mostly by FOMO).

FB was trying to refute that, and their study was more technically rigorous, but the outcome they were measuring doesn't actually refute the original claim at all IMO. People writing more happy posts on FB does not indicate they are actually happier, it could also be that they wish to broadcast more of their happy moments, or perhaps even just pretend to be happier, in response to the attitude of their feed.