It's more explicit now. Most sites add the Facebook Pixel as they want to track conversions on their site. Most brands have this on their site. Facebook can do all of the analysis that Google can with that, such as knowing how many people add something to their cart, checkout, etc.

The power is in combining that data, like one of the grandparent comments mentioned. They will know if you buy a SUV (since the dealer uploads the data as an offline conversion) and that you're browsing baby clothes on Walmart/Target. Now you're a "new or expecting parent".

Some of these interest categories are not that explicit but the lookalike audiences are. That's a whole other post :)

What do you think Google Analytics does?