This means that Facebook will not only be able to monitor how you use its site, but also other sites who opt in. Data regarding your page visits, how long you spend reading posts, and what links you click can be recorded on third-party websites. However, Facebook can only offer tools to the owners of third-party websites; they will have to sign up and implement these tools.
Of course, you don’t have an option to opt in or not. You were automatically enrolled as of January 30th. It seems that you can opt out through Facebook's mobile apps. This may only affect the ads you see rather than the data that’s tracked.
The EU's privacy watchdog isn’t happy with these changes. It argues that users should have the choice to opt in or not. It’s likely that many users will see it the same way, but will it be enough to dissuade them from using the social network?