Today is April 30th, the original deadline for Sign In With Apple compliance.
While Apple has pushed back the date to June 30th making Sign In With Apple mandatory for updates to existing apps that include social sign ins, all new apps must comply as of today. No new apps will be approved without it.
Over the next 2 months, we're sure to see all the major apps and independent developers supporting Apple's sign in method. I'm not aware of a master list of apps that use Facebook's sign in method, but I assume that it's very very extensive. As of June 30th, all of them must comply.
Sign In With Apple will become as ubiquitous as Facebook Connect which was built steadily over 14 years while Apple’s adoption will be compressed into less than a year.