While I'm sympathetic to your viewpoint and to a large degree share it, you are among a vanishingly small minority to not use a site or service just because of social login. 'jfaucett probably is fine sacrificing losing this small minority.
precisely. In my experience, this small minority can actually be subdivided broadly into two groups: those with very high data privacy concerns and scammers/spammers. It turns out that in my experience, the high level data privacy concern people have been the vastly smaller of the two groups, so at the end of the day you are sacrificing a tiny fraction of a fraction of potential users while simultaneously removing tons of scammers from your platform. Admittedly, the ratios probably vary depending on the particulars of your pricing models and potential scam vectors that can be applied to your software, also other factors such as local attitudes toward data privacy (my experience has been in the EU). You can also increase sign-ups by a large portion of the data privacy people by simply doing a lot of things to make your services legit i.e. have domain names registered to the real business or contact person, force https, have a high level overview of your terms and services that clearly and succinctly states how you use users data, have a verified https certificate, etc. But in general "social login only" has been so much better for me than mixed or only email logins I really would not want to go back to the alternatives.
However, I login with Google all the time.
I have my reasons for these choices and I'm sure many people here share those reasons, and I'm also sure many more don't share those reasons, but the reasons are not important in this discussion.
If you're going to offer social login, then offer at the very least all the major social logins - some really are more acceptable than others, the problem is, everyone's set is different.
Note that ‘major’ can be different for different countries, regions, markets, product types. So be sure to know your target audience.
If I'm VERY interested I'll consider google or github or something, no matter how interested I am I'll never use facebook or twitter etc.
How do you test this? I'm not sure how to test this though as there are flow-on effects on preventing registrations ie less word-of-mouth.