Facebook still has a complete image of the things you like, your social graph, intimate details about your marital status, mobility history, personal messaging and so on.
They can still build profiles orders of magnitude more relevant than, say, Google. The web tracking is just an optimization to get the types of data Google has, too.
Most of their advertisers use the pixel to retarget people.
These are by far the most profitable ads for advertisers, and often play a part in a sales funnel in which advertisers may lose money on initial traffic acquisition but make it up and then some through retargeting. Without the pixel, I don't care how many pages as I have "liked," how well they know where I've been, or how many data scientists they hire, they'll never be able to extrapolate that I was considering buying a specific brand of coffee maker at a specific site recently and be able to show me an ad for it.
Retargeting is not just an optimization. For many advertisers, it's the saving grace of the Facebook ads system that produced absurdly poor results before it was introduced. Facebook ads were the brunt of jokes back then, and it will return to that status if too many people use this feature.
They can still build profiles orders of magnitude more relevant than, say, Google. The web tracking is just an optimization to get the types of data Google has, too.