Yes it's right that Apple have stopped the Facebook log in option with their latest iOS updates but, if you've got an old iPad (or iPhone), you should still be able to log in via Facebook. I've got an iPad 2 and an iPad Mini both of which stopped updating at iOS 9.3.5 (the latest iOS for newer models is 11.2.1) and have logged in successfully to the Repertoire app.
The Repertoire app, as far as I can tell, only shows new uploads and there's no easy access to what must be a huge database of registrations users have uploaded. You can search for a particular song but that gets a bit tiresome as the ones you want don't seem to be there (well not the ones I wanted anyway). The best way I've found to search is to enter an artist name eg 'beatles' and that will produce a number of results you can pick from. Also, if you're feeling adventurous, you could try eg 'night', 'love', 'man', 'sweet' etc etc to produce quite a number of random songs with those words in the title.
I think it's a brilliant idea and it's fascinating to watch the registration transfer to your keyboard but, as Arno said, there are some poor quality registrations.
However, there are also good ones. 'Come Fly With Me' from a Dutch uploader is particularly impressive!
Chris