Re:What Do You Think about This Seamless “Registering/Login”...
by netsharc on Sunday November 03, 2019 @02:42AM
(#21432668)
Well FB (and Twitter) already have "Like this page" or "Tweet about this" embeds, and those allow them to track you across the Internet (or at least on sites with those buttons).
IMO a website must ask for a permission before displaying this button.
Facebook also has a similar button ("Continue with {my_name}") that use the same iframe method.