One thing that has been really interested about the discussion of social logins has been the re-emerging critical outlook on online identity. I think that social logins are a double-edged sword, where they give us the ability to easily connect with sites for which our social identity is relevant or for which setting up a whole new custom identity is unnecessary. One the other hand, the obvious drawback is the implicit promotion of the social network as the de facto identity standard, which is dangerous and totalitarian (Facebook owns who you are, sort of).

I think the simple value for social login is context. There's an obvious overuse case and a useful use case.