Facebook used to look at your activity on Facebook and store this and analyse this and use this to show you 'relevant' adverts.
As of now, Facebook admits to looking at your activity across various websites, stalking your every move in an attempt to get to know you better, and to show you an ad that you might just click on. Maybe. One day. After all, they need to make money somehow.
For a change, the PCWorld article about this change is actually less Facebook propaganda heavy than the Slashdot one. It notes the 'extra control given' that Facebook is claiming, but also points out to what extent Facebook is actually watching your activity on the Web:
This Slashdot contributor however just buys the propaganda completely. You can 'opt-out'. Isn't that reassuring.
It turns out, not so much. You can opt-out on a specific device, using a specific browser. All this does is store a cookie, and Facebook then promises to not use information gathered while the cookie is set to give you adverts (although if they did, it would be very hard to tell). If you use another device, a different browser on the same device, or if you clear your cookies, you'll have to 'opt-out' again.
And you don't opt-out of the tracking itself. Oh no - just out of allowing them to use that information. Nothing about actually storing it. See here to read what I'm talking about and to press that opt-out button if it makes you feel better: https://www.facebook.com/ads/website_custom_audiences/. It'll probably feel a bit like pressing one of those pedestrian buttons at traffic lights in this country: it might not even be wired to anything.
So again, if you want privacy, stay off the 'Net. You're not going to find it around here.