A specific website for service designers to share their work
The Service Design Network has a section where you can share your own case studies. This is maybe the one specific site for service designers to post their work.
Design platforms to share your service design work
There are platforms made for visual work that work great also to present service design work.
Website builders with great portfolio features: Squarespace, Semplice, Carbon Made, Adobe Portfolio, Wordpress
Platforms made for any types of professional
Obviously, outside of this platform, the usual players that are true for many professions are also true for service designers:
Linkedin