If you're someone that uses Skype by signing in with your Facebook account, you're going to want to change your habits. Microsoft is killing off support for the feature in January 2018.
Obviously, the firm would rather have you sign in with a Microsoft account (MSA), which can be used for all of the company's services, like Office 365, Xbox Live, and more. Microsoft is also making it easy to get started; if you've already got an MSA, you can follow these instructions to transfer your contacts.
At this point, there can't be more than a handful of users to still use Facebook to sign into Skype, as it doesn't work on many platforms. You can only do it on Skype for desktop and Skype for Web, leaving all mobile and Windows 10 clients unable to do it.
If you are one of the few that still use Facebook to sign into the service, you've still got a couple of months to get started with a Microsoft account, although you probably shouldn't wait that long.