At least since the last time I uploaded to it, Sketchfab has its own engine, things directly out of Blender will not work. It will upload your model and read the material slots, but not the node editor and its nodes. In other words you have to make your own shading with the tools that Sketchfab offers. You can also bake your textures out of Blender and upload them into your materials in Sketchfab. Try it first with a simple model so you can start getting the hands of things.
