As of Substance Painter 1.5.0, exporting to Sketchfab is natively integrated! If you purchase or upgrade to the latest version, you can already upload your models directly from Substance, complete with PBR textures!
How to use
Open your 3D model in Substance Painter.
Go to File → Export Textures...
Choose Sketchfab from the Output template drop-down menu.
Log in to your Sketchfab account.
Enter a title, description, tags, and mark it as Private if necessary.
Check Update existing model to re-upload/replace a model on your Sketchfab account.
When the upload is complete, you can view your model on Sketchfab, adjust rendering settings, and save the default view. You can also open the local folder with your exported files.
For more detailed instructions and other information, visit the blog or watch the video above.