I encountered the "which method did I use to register to this site" problem years ago and I have a simple solution (as a user) to this: When there are multiple

I encountered the "which method did I use to register to this site" problem years ago and I have a simple solution (as a user) to this: When there are multiple one-click login options, I register with Facebook first, if that's not available or doesn't work, I use Google, then Twitter. So I don't have to remember which service did I used, just stick to my simple rule and login with Facebook first.