[priansh]

They have their own pixel that presumably helps them match users to visitors; also most sites have a Facebook like button somewhere.

The FB app ID is also one of the most common meta tags behind Open Graph.

A nice small example of what this can do can be seen with Clearbit [0] which does a good job of telling you where someone works based off of nothing but their IP address. Imagine that but with the exponentially larger data warehouses of Facebook or Google, paired with referrer tags (FB has CLIDs that allow them but not you to match clicks to actual users) and meta tags (FB can tell exactly what app, page etc a website is associated with and use that data to advertise to users).

[0] https://clearbit.com

[coldcode]

Since I started using Safari and iCloud Private Relay, hiding my IP address, this no longer works, since the IP they see changes a lot and is only vaguely in your area. I think FB etc. knows the address range of the IP that are exposed and just gave up using them as it gives no useful information.