Still Yahoo Mail is the mostly used email service , Yahoo wants to attract more users to use their Yahoo network by allowing Facebook and Google users to sign in to Yahoo with their Facebook and Google IDs.
To access Yahoo Services you don’t required to have a Yahoo account instead Google or Facebook account is enough, Yahoo delivers same experience on Yahoo whether you sign in with Yahoo, Facebook or Google account.
Logging into Yahoo with Facebook ID
Logging into Yahoo with Google ID
This week Facebook or Google ID sign-in options for Yahoo will go live.