Android
One account can be bound to:
  1. Email
  2. Google account
  3. Facebook account
You can bind your account to an email address, a Google or a Facebook account at the same time. But one email address (or Google and Facebook account) only corresponds to one account.
iOS
One account can be bound to:
  1. Email
  2. Game Center account
  3. Facebook account
You can bind your account to an email address, a Game Center or a Facebook account at the same time. But one email address (or Game Center and Facebook account) only corresponds to one account.