In general, tech people are more likely to be early adopters. Also, it helps if the VCs you're targeting can log in as well. Same reason most mobile startups are iOS well before they're Android.
In general it goes (GitHub if you're a dev tool, Apple if you're mobile), Google, MSFT, other SSO, in that order.
In general it goes (GitHub if you're a dev tool, Apple if you're mobile), Google, MSFT, other SSO, in that order.