I think I found a some workaround. A little complicated, but it works. Maybe for someone this information will be useful. So:
1. model created in FreeCAD was exported as .STEP file
2. .STEP file was imported to 123D Design
3. from 123D model was exported as .SAT file
4. .SAT file imported to OnShape, and next was exported as .dae (Collada) file
5. .dae file can be uploaded to SketchFab
6. (option, instead of step 5) .dae file can be imported to Blender (eg. for animation) and exported directly from Blender do SketchFab
This method gives smooth surfaces and keeps the information about the elements and colors/materials. Of course, you can start from any step, eg. from OnShape, depending on which program you likes to work.
All of the above steps uses only free software.
You can see the difference:
- Direct export to VRML with FreeCAD SketchFab https://skfb.ly/XzYL
- Export using the method described above https://skfb.ly/XBTx
If anyone knows a simpler way to get smooth objects from FreeCAD in SketchFab let share this information, please.