tl;dr Facebook tracking pixels placed on websites around the web are automatically loaded by popular browsers' default settings. These benefit Facebook and their customers (advertisers, data brokers) but have created privacy risks for users.
Solution for users: Block loading of tracking pixels, e.g., via browser extensions, DNS, filtering proxy, etc.
They can also match on personal information - tracking pixel or not, letting you know if user X, of which you know any piece of information, is in the target audience or not. Specify the target audience sufficiently, and you can find out what facebook knows about user X.