From 2014:
https://www.theguardian.com/technology/2014/jun/29/facebook-...
> now Facebook, the world's biggest social networking site, is facing a storm of protest after it revealed it had discovered how to make users feel happier or sadder with a few computer key strokes.
> It has published details of a vast experiment in which it manipulated information posted on 689,000 users' home pages and found it could make people feel more positive or negative through a process of "emotional contagion".