As from 10th January 2018, you will not be able to use your Facebook credentials to sign in to Skype.
Facebook sign-in will not work on any Skype platform: web, desktop or mobile. The Facebook account transfer tool that was provided to help with this transition has also been decomissioned.
To continue using Skype, either create a new Microsoft account or sign in with an existing Microsoft account if you already have one. Learn more.