I tried the module LoginToboggan to log in entering the email and not the username (that I use as Facebook ID). The problem is that the module allows to log in with either the email or the username.
How can I log in with just the email?
There is a module for that, try Email Registration, from module description:
Allow users to register and login using only an email address. Users can then log-in using their email address and password for authentication.
A username is generated and assigned based on the user name part of the email address and their user ID
is avilable in D7, D8