Ask your same question about any other instance.

For example, what’s keeping me from setting up my own instance that just spams ads across every community? Answer: nothing.

What could owners of instances do to prevent seeing advertisements? Answer: defederate.

How much are users of instances that don’t defederate from my spam instance tracked? Answer: the same as any other site.

Tracking online works by communication with a client and server, ideally with identifying information. Everyone on the web has some identifying information, such as IP address and user agent. The problem is, IP addresses are shared with many people, the same as user agents. The only way to uniquely identify a user online is with cookies. Facebook used 3rd party cookies to track users who visted site A when site A has a image serve from facebook servers. Facebook gives the visitor a 3rd party cookie “You are now 93ga3490f” and logs that the cookie was served to visitor 93ga3490f on site A. That visitor then goes to site B and sees a facebook like image button, but this time when the user asks for the like button from facebook, the browser says “Hello, I am 93ga3490f requesting the facebook like button” and facebook records that 93ga3490f also visited Site B. Honestly this is still pretty useless until that user logs into their facebook account. At this point, the browser tells facebook, “Hello, I am 93ga3490f logging into facebook with email xyz@example.com” and facebook records xyz@example.com as visitor of Sites A and B (formerly user 93ga3490f.)

How does tracking work with federation? Answer: it doesn’t. Federation you can think of as a server subscribing to another server. You’re offline, and instanceXYZ downloads a copy of new content uploaded to instanceABC, since instanceABC and instanceXYZ are federated with one another. You go online, log onto your instance (instanceXYZ) and instanceXYZ serves you the downloaded content that it previously got from instanceABC. instanceABC doesn’t know you’ve viewed this, doesn’t know you’ve downloaded this. All instanceABC knows is that instanceXYZ copied this content for all of its users. The only way you can be tracked here is:

1. instanceXYZ decides to track what it serves to you.
2. OR instanceXYZ decides to include a facebook like / share button for each post AND you are using an outdated web browser to use lemmy AND you have a facebook account.