I think adding social auth was one of the biggest mistakes we made for the beta of Jetstrap. Users constantly forget which service they use. We added email/pass authentication, but now older users think they had a password when they didn't, and with django social auth you can't reset the password for a social account.
It was one big mess not at all worth it. I'd like to try Persona and see how that compares, but I think normal email/pass is better than plain social auth.
Why don't put the way the user was logged in by last time in cookie and just highlight it next time for him? Or even more explicit - add the "you have logged in this way last time" under the appropriate button?
If a user returns relatively quickly they generally remember how they logged in. The problem is when they come back after some months. At that point, they've probably completely forgotten how they logged in, and have cleared their cookies for some reason or another. It's a standard troubleshooting suggested by many websites. (Not saying it should be so, but it is.)
It was one big mess not at all worth it. I'd like to try Persona and see how that compares, but I think normal email/pass is better than plain social auth.