Quantifying the impact of offering alternative signup/login methods?

Nowadays it’s common for websites to offer signup/signin methods that don’t require making an account (email/password) for that specific site, for example ‘Login with x’:

  • Facebook
  • Google
  • Twitter
  • LinkedIn
  • etc

By how much does offering each common alternative signup method increase the signup rate for an average website, as opposed to just offering traditional email/password signup?

Notes:

  • Obviously these statistics could differ (a lot) from site to site, depending on the demographic and other factors. But I currently have very little idea of how much impact each signup method has, so any information is a massive improvement on that

  • Very interested to know if any studies have been conducted, or if any companies (big or small) have released statistics.

Example

Suppose a website only offered the traditional username/password signup method and it signed up 1% of the visitors to the site, but then it got a ‘Login with Google’ feature, and its signup rate went to 1.1%, then we could very roughly guess the ‘Login with Google’ feature increased the signup rate by 10%. (figures completely made up)