Privacy Badger is EFF's free privacy plugin; it blocks trackers and ads from companies that practice "non-consensual tracking," in which your browser's "do not track" instructions are not honored.
That means that advertisers who respect your privacy choices still get through, but everyone else is blocked from ever seeing you. And Facebook is a major customer of those "nonconsensual tracking companies" that hoover up your data as you traverse the web and then sell it to Facebook to add to the dossier that Facebook compiles on your every motion.
I use Privacy Badger, and it's a big part of how I defend myself against the kinds of data about me that Facebook can provide to creepy companies like Cambridge Analytica.
Facebook collects data from two sources: first-party data (things you do on facebook.com) and third-party data (including data collected overly or covertly via the "Like" button on other websites, "Login with Facebook," Facebook Messenger chat widgets, and the Atlas advertising network that Facebook owns). Even when you do not consent to share your information within an app or service, your data may be collected and traded by third-party trackers and data brokers.
Facebook is just one example of a much larger problem: online platforms and companies overwhelmingly operate with a surveillance-based business model that relies on gathering as much information on users as possible. This means companies gather data about you not only when you visit their site or service, but also follow you around when you move around the web. These third parties are often shadowy data brokers or advertisers most users have never end heard of.