If users sign up through a social site, their account is still identified by email. So as long as their email addresses are the same between sites, it doesn't matter which social service they use, so they won't have to remember. Twitter doesn't provide an email address, however, so that could be a problem.
yes, you're right about twitter. However, we don't have that much of a problem with people hitting the social login button as much as with people forgetting that they used social last time they logged in.
They then try to enter email and password. When it rejects them, they try to reset the password, and get frustrated that the reset password does not work...
We relatively regularly answer support emails and just tell users "It looks like you used your google account to sign in last time, just hit the right button and you'll get in instantly"
Not only that, Facebook's default privacy settings are set up in a way which most users' proper email address is not exposed to sites using social login.
