You can reach out yourself directly via email or via social media to all of your followers and clients and tell them you are moving your content to Thangs.com. If you tag @Thangs3d we will help you get the word out to your following.
Shapeways no longer has your client information used in the previous marketplace. Shapeways however does have a generic list of community members that we will inform about our open invite to our former shop owners to set up shop at Thangs.