I understand the security issue related to the social network accounts as it relates to an email address and can appreciate the security issues that can come up as a result of trying to link social accounts with WordPress accounts.
However, in the case of Twitter, LinkedIn and WordPress the email address used for each account must be verified before an account can be created, so why does this not pull across when connecting with your plugin?
I have Facebook, Twitter, Google, LinkedIn and WordPress.com accounts set up. All of those accounts use the same email address I use for my WordPress website (http://bamajr.com/). With all these accounts, except Google (which works fine), I even have the same username (bamajr) and I use the same First/Last name.
When I log into my WordPress website using your plugin and my Facebook/Google credentials, I'm matched with the correct profile, previously created, on my WordPress website.
When I log into my WordPress website using your plugin and my Twitter, LinkedIn and/or WordPress credentials, I'm not even able to get to the dashboard nor see my user profile. If I try, I get "You do not have sufficient permissions to access this page." If I then log out of my WordPress website and log back in with Facebook/Google, or even the account I previously used without your plugin, there are users created with really long (hash-like) email addresses and no first/last name.
It seems to me if my WordPress website, Facebook, Twitter, Google, LinkedIn and WordPress.com accounts are all using the same email address, username and first/last name, then I should be directed to the correct user profile when logging in with your plugin. Otherwise, what is the point in matching user profiles in a WordPress web site with those of social networks?
And again I ask, if it works correctly with Facebook/Google, why isn't it working for Twitter, LinkedIn and WordPress, given the information I've provided in this reply?
RE: Login Email Sync Issue...
The plugin is supposed to "SYNC" the username and email address for the purposes of logging in, but is not supposed to replace a username with an email address. Its supposed to allow a person to log in with either one.
However, I have been able to do further testing and prove it is unrelated to your plugin and have notified the plugin developer as such.
The only reason it only came to light when configuring your plugin, is that a user logged in to my WordPress website with social credentials (I.E. Facebook/Google). They went to their user profile on my WordPress website to make sure nothing had changed within the profile. Instead of exiting the profile they clicked the "Save" button. This is actually where the issue is. A user can use both username and email address, with the "Login Email Sync" plugin, until they update/save their user profile. Then they can only log in with email addresss.