Info: Settings - Features - Unlisted and Private Videos / "You may change your videos' privacy settings"
"User Favorites Playlist is no longer public accessable. If you want to share your favorites with others then you can create a User playlist, set it's permissions to Public, and then add your Private Favorites to your public playlist.
And also if you want to make it known to your friends when you add videos to your Public playlist - then you make sure to enable that option in your YouTube Account Settings - > "Sharing""