I'm pretty sure it's not about it being hard to implement, but also about reducing friction to sign up/in. I'd personally never use sign in with Facebook, but

I'm pretty sure it's not about it being hard to implement, but also about reducing friction to sign up/in. I'd personally never use sign in with Facebook, but for nearly all dev related services I prefer sign in with GitHub over creating another account somewhere. It also reduces the burden on security, you no longer have to think about where to store the usernames and passwords, how to hash them, how to compare the hashes to the plaintext password in constant time, how to prevent brute force attacks, how to prevent csrf and more. There is a reason that the sign in with X sign up flow is so popular!