Thanks for all your comments. I've made changes based on Shal's suggestions:
If you register on the website with a password, social logins are
disabled by default.
If you register on the website using a social login, that social login
is enabled and all others are disabled by default.
If you register through email, a direct add, or an invite through
email (i.e. don't have a password set), you're in a special state,
where the first time you log into the website, your social login
permissions are set:
- If you login using a password (going through the password reset
system), social logins are defaulted disabled.
- If you use a social login to login, that service's social login is
enabled, all other social logins are disabled.
For everyone, in your profile, you can now toggle Facebook and Google
social logins on or off individually.