If you run a site that uses Facebook for login, when you log someone in via Facebook, you can ask for their email address via the "email" extended permission. Your site can then support a password reset page (like most sites already do) to enable those users to create a password via email if they decide to delete their Facebook account. Requesting email address is a good "escape valve" for users if you are concerned about this aspect of Facebook login.
If you run a site that uses Facebook for login, when you log someone in via Facebook, you can ask for their email address via the "email" extended permission. Your site can then support a password reset page (like most sites already do) to enable those users to create a password via email if they decide to delete their Facebook account. Requesting email address is a good "escape valve" for users if you are concerned about this aspect of Facebook login.