To give you “suggestions”, Facebook sends everything you type in a status update to its servers (so they know what you’ve typed, even if you don’t submit the status update itself). Watch the screen capture below to see how Facebook are doing it:
In the past, Facebook released a study where they analysed exactly this information.
We only know of that study because Facebook released it. Given the negative feedback that study and the emotional contagion study got, they probably won’t be making the mistake of publishing such studies in the future. They run such algorithmic studies on users all the time, of course, because it is their business to do so (see ind.ie/the-camera-panopticon).