1) Is Google or Facebook user ID number considered personal data? The ID itself is just a number, but you can use it to construct a profile url and reveal his identity through Google or Facebook profile.
2) If I have a website which enables social login and therefore stores registered user Google and Facebook ID numbers in the database, with no other personal data, not even email or name, just these IDs, am I data controller?
3) If answer to 1) is "yes", then is the user ID number in salt-hashed form considered personal data? Having a hash you cannot convert it back to user ID. But having a user ID e.g. from his social profile, it's possible to convert it to a hash and compare against all hashes in the database to tell if this person has account on this website or not.