The Electronic Frontier Foundation, a nonprofit focused on privacy, internet transparency, and free speech, has released a fantastic article on how to use Facebook's new Off-Facebook Activity tool. This Off-Facebook Activity is exactly what it sounds like, it's all the data Facebook receives about your interactions you have with other entities, such as an app on your phone or a retailer you shop at. This is a big part of how Facebook can serve ads based on weirdly specific desires you've expressed elsewhere.
Spooky, right? Well, here you go, courtesy of the EFF. Go donate to them.
1. Navigate to the “Your Facebook Information” section of your settings and select “Off-Facebook Activity.” (Or, just go to https://www.facebook.com/off_facebook_activity/ while logged into your account.)
2. You’ll see an option to “Clear History" in the sidebar on the right, but we want to use a more powerful feature. At the bottom of that sidebar, click “More Options” and then select “Manage Future Activity.”
3. You’ll see a pop-up giving you some more information. Click “Manage Future Activity.”
4. Click the blue toggle.
5. Click “Turn Off” in the pop-up, and you are done! This should return you to the same screen, with the toggle now grey and to the left in the “off” position.
And that's it! You're done. Now it doesn't stop OTHER 3rd parties from collecting information on you, but at least you're getting less stuff sent to your FB.