Every time you go to your Facebook and access the news feed, every "Like" to a photo and with every moment that you use Messenger to connect, you increase the Facebook's knowledge about you and your personality. Consequently, that is a small and meaningless dot within their wide universe of data about their billion-plus users.
Plus, Facebook just keeps extending its remit of data. Presently, you can search all of its trillions of posts - meaning that it can and will increase more and more search data to what it already knows.
It's possible that Facebook brags about having the widest, inmost, and most complete dataset of human knowledge, interests, and activity that has ever been collected. Google, on the other hand, presumably has more raw information, between Android and searches-but the data that they gather is (mostly) much less personal. It's legitimate to say, that Facebook almost unquestionably knows you best than any other social media platform.
They can use this data for advertising, which is argumentative. What's long been more interesting is the probability of entering from this data, i.e. concluding from your online behavior things that you never obviously revealed to Facebook - i.e. anticipating your reactions to new data and situations. What's more interesting is the concept that Facebook might be able to create a unique and authentic picture of you, with all the information points you give it as the pixels.
Although that idea pretty abstract. Let's try a couple of detailed examples. Facebook can find out with a high degree of certainty, from the way that you use its app and site, from the links and pictures you post, and the stuff you've "Liked", whether you're a dedicated person or a idler, and whether you're a good or bad credit/insurance threat.
