Starting in January 2018, Microsoft says (via Thurrott) you'll no longer have the option to sign in to Skype with your Facebook account. Instead, all Skype users will be required to sign in with a Microsoft account.
The change comes as an effort to "streamline" all Skype logins around your Microsoft account as Microsoft continues to roll out its new Skype experience, the company says.
Using a Microsoft Account will enable you to login-in via single sign-in, unlocking all your Microsoft services – including Skype, Office Online, Outlook, OneNote, OneDrive, and more – in one experience, with just one password. Currently, using your Facebook account to sign in to Skype is not supported on Skype for Windows 10, the new Skype for Android and iPhone, and the new Skype for Windows, Mac, and Linux.
Given that support for signing in via Facebook was already fairly limited, it's likely this won't hit too many users. Still, it's bound to be an inconvenience for some. Fortunately, you can transfer your Skype Credit, number, and other account information to a new Microsoft account, and Microsoft has outlined how.