Add Social Media icon widget to customizer

Your widget is okay but problem is customizer do not show sidebar option if there is no sidebar is called on current viewing page.

You did register the two siderbars but did not call them any where in the template of page your are seeing in customizer.

Use dynamic_sidebar function and call your sidebar somewhere in the template.

Example:-

dynamic_sidebar( 'smb' ); //For social media buttons sidebar
/* OR/AND */
dynamic_sidebar( 'lb' ); //For link button sidebar

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