Is Sketchfab just for files from Sketchup, or can it be used with other programms.

Does Sketchfab has advantages compared with 3D PDF.

Rudl

Sketchfab is online 3D sharing system for view only, just like Youtube for the video.
As you can see, it can be embeded in a web page, and use webGL standard.
The model can be uploaded directly from some 3D soft like Sketchup, 3Dmax, C4D, Blender... with a plugin (not MoI :( ) or via the web undersantding 25 3D format (polygonal, so no 3DM :( again).

<< Does Sketchfab has advantages compared with 3D PDF
an infinite free direct public data base like 3DWareHouse for Polygonal or GrabCad for the CAD (nurbs...) ;)
http://sketchup.google.com/3dwarehouse/
http://grabcad.com/

<< not MOI>>

But you can export some objects in polygonal format from MOi: see in the list following these with !!!
So no need to return in Sketchup for Sketchfab! ;)
> Blender (.blend)
> Collada (.dae),
> Wavefront (.obj) !!!
> FBX (.fbx), !!!
> Design Web Format (.dwf),
> OpenSceneGraph (.osg, osgt, osgb, ive),
> 3DS (.3ds), !!!
> Lightwave (.lwo, .lws), !!!
> Polygon File Format (.ply),
> Virtual Reality Modeling Language (.wrl),
> Open Inventor (.iv),
> Shape (.shp),
> Standard Tessellation Language (.stl), !!!
> Biovision Hierarchy (.bvh),
> Open Flight (.flt),
> ac3d (.ac),
> DirectX (.x),
> Designer Workbench (.dw),
> 3DC point cloud (.3dc),
> carbon graphics Inc (.geo),
> Generic Tagged Arrays (.gta),
> Keyhole Markup Language (.kmz).

One more... Cleaner, but with more polygons...