Sketchfab is all about community. Beyond the option to comment on models, PRO users (and above) can interact with their followers or fans through the Contact button on their profile.
Once you click Contact, you'll be asked for your name and a message title and body.
After completing this form, Sketchfab sends an email with your message to the Sketchfab PRO (and above) user.
If you contact a Sketchfab PRO or above user with this option, your email will be revealed to them. You will not see their email address until they reply.