Microsoft has confirmed that they will disable logging in via your Facebook Account into Skype in January 2018.
The feature is already legacy, with only older versions of Skype supporting it, while the Modern and latest desktop version and the versions for iOS and Android already demanding that you use your Microsoft Account.
Microsoft says the move is to “streamline” skype logins, putting it on the same footing as Office Online, Outlook, OneNote, OneDrive.
Microsoft said they would facilitate migration from the Facebook to Microsoft Account, with instructions on their support site here.
While most regular users of Skype would not be affected by this, having probably already upgraded to more recent versions of the app, I suspect some older users with older PCs may be frustrated by being forced to get a rarely used and probably otherwise superfluous Microsoft Account.